How Diamonds are Formed

This shows the Earth's mantle where diamonds are formed.
Courtesy of the Debeers Group.

A vast amount of knowledge has been developed since the 1980s about how diamonds were formed.  It has been through the inclusions found in diamonds that much of the information about how they were formed and their origin was discovered,  Inclusions are all natural materials that are found in diamonds including some that are gaseous, liquid and solid inclusions.  Although theoretically it is possible to form diamonds by squeezing a lump of coal in nature that hasn’t happened although in the lab this had been done under conditions of extreme heat and pressure in giant hydraulic presses,

Some of the diamonds that have been found in meteorites that are more then 4.25 billion years old far exceeding the age of life on Earth.  However natural diamonds on earth were formed at least 990 million years ago.  One of the discoveries made since the 1980s shows that diamond is formed whenever you burn acetylene gas that forms as a coating on metal that has been exposed to burning acetylene that is often deposited during the course of oxy-acetylene welding.  This type of diamond coating has since been made use of industrially for imparting a hard coating onto metals.

How a diamond bearing pipe is formed.
Courtesy of the Debeers Group 

It takes extreme pressure that ranges from 40 to 50 thousand times higher then the pressure found at the earth’s surface of 14,7 pounds or 1,000 millimeters that is termed one bar.  This is the pressure encountered at depths ranging from 125 to 200 kilometers below the surface   Most diamonds are formed from kerogene that was deposited in vast quantities on earth during its formation.  There are other diamonds that were formed from carbonaceous material like fossil coral or shells that have been subducted deep into the earth’s crust where they were also exposed to extreme heat and pressure.  A few diamonds were even formed at depths ranging from 300 to 400 kilometers deep or even further, but these diamonds are extremely rare.  Most diamonds were formed at temperatures that exceeded 900⁰ to 1,300^o Celsius.

Formation and Emplacement of a Kimberlite Pipe

At about 125 kilometers beneath the surface of the earth the pressure of the magma cracks the surrounding rock until the cracks reach the earth’s surface.  Much of this cracking is due to contained carbon dioxide in the magma that causes the magma to bubble and expand.  This is caused either by heat from below or reduced pressure as the magma column reaches towards the surface.  It is this expansion that eventually causes the magma to erupt explosively at the surface.

An eruption like this has never been observed by mankind because it has been millions of years since one occurred on earth predating mankind.  It has been theorized that an eruption of this type rather then being hot like most volcanic eruptions are; it is emplaced under freezing temperatures because of the rapid expansion of the volcanic gases.  In the final few kilometers these gases rise at the speed of sound where they form a pipe following the path of least resistance through the surrounding rock finally reaching the surface.

It is because of this rapid ascent of magma that allows the diamonds to reach the surface of the earth rather then being converted into graphite a far more stable substance at the earth’s surface.  The pressure him pardoned by dissolved gases in the magma eject the diamonds they don't have time to be degraded into graphite.  The diamonds are actually xenoliths, particles caught up in the magma train as foreign objects.  Once the volcanic cone cools it starts too erode releasing the diamonds into the soil. The diamonds that are not eroded state trapped in a kimberlite or lamproite where they are mined.

Different Types of Diamond Deposits

How diamonds are mined.
Courtesy of the Debeers Group.

Diamonds are found in two different types of deposits of which primary deposits come first.  These are deposits found in kimberlite, lamproite or komatiitef pipes and are mined lake any other mineral found in hard rock.  Secondary diamonds are found in river gravel deposits where they are concentrated after traveling for some distance from their primary source.  The gravel found in the Orange River in South Africa is an important source of diamonds.

Often overlooked by many is a third source of diamonds found in a rock type called conglomerate that are really fossilized river gravel.  One important formation like this is found near Marathon, Ontario that not only carries diamonds, but rubies and sapphires as well..

Appalachian Diamonds

A diamond crystal in matrix
Photo by Rob Lavinsky

Look out, alluvial diamonds have been found scattered along the backbone of the Appalachians for years making it likely that sometime in the future a lucky prospector is likely to find a kimberlite or lamproite the two types of rock where diamonds are apt to be formed.  Diamonds have been found in Virginia, West Virginia, North and South Carolina, eastern Kentucky and Tennessee, Georgia and Alabama.  All of these states are below the area that was affected by the continental glaciers of the past million years making it likely that their host rocks are in place.

At the time of its discovery in April 1928 the famous “Punch” Jones at 34.43 carets was the second largest diamond ever found in the United States.  The Uncle Sam diamond was found in 1924 at Murfreesboro Arkansas in the Prairie Pipe Mine weighing at the time of its discovery 40.23 carets is the largest diamond ever found in North America.  The Punch Jones diamond is presumed to have washed Rich Creek from neighboring Virginia.   There are five sites in or near to Virginia where diamonds have been found one in kimberlite and one in peridotite; it is presumed all of these diamonds came from similar rock types.

Brilliant Cut Diamonds

North Carolina produced 13 diamonds in the mid-1800s.  The North Carolina Geological Survey has recently undertaken studies of lamproites in the Charlotte area to better understand where these diamonds originated and to discover clues as to their origin for other exploration projects in the Piedmont.  The other Southern States produced one or two additional diamonds.  The lack of glaciation in all these states suggests a local origin for theses diamonds.  Lamproites and kimberlites have been discovered in the immediate areas where most of these diamonds have been found, but none have been found since WW II when gold mining operations ceased in the southern Appalachians.

Resumption of placer mining operations make the discovery of diamonds likely.  Most of the diamonds discovered in the past were from placer mines with a few exceptions.  The Punch Jones diamond is one of these exceptions it was discovered when Punch Jones and his father were playing horseshoes.  Although this diamond was discovered in 1928 it took until 1942 before its true identity was established.

Fortunately diamonds have some distinct minerals associated with them making it possible to find diamond deposits by tracing the diamonds and other associated minerals to their source.  These minerals include type “G” pyrope garnets, chrome diopside, phlogophite mica and serpentine.  The mineral illmanite is frequently found associated with diamonds too.  Tracing these minerals is often done with soil samples taken below the “A” horizon that contains organic matter.  

Gemstone Occurrences in New York:

Herkimer Diamonds from the Ace of Diamonds Mine.
Photo by Rob Lavinsky

You could almost write a book just about gemstone occurrences in Manhattan that has produced beryl, chrysoberyl and some huge garnets.  These are all gems that have been produced from excavations and the rock removed from them.  Most of this rock now resides along the bank of the Hudson River in uptown Manhattan where you can readily get at the excavation spoils from Inwood Park.  The entire southeast part of the mainland has the same kind of gemstones as Connecticut with whom it shares a common border.

Saratoga County has produced beryl and rose quartz from the pegmatites found in the Adirondacks.  Almandine garnets have been produced from a deposit on Gore Mountain in the town of North Creek..  These garnets are embedded in a matrix of gabbro with some of the crystals reaching more then a meter in diameter.  The Barton Mining Company produces the garnets for making abrasives, but sufficient amounts of cuttable material is produced to satisfy the needs of the Rock Hound trade.

In the middle of a city park in Syracuse is a kimberlite pipe that may be diamondiferous or not depending upon who is telling you about it.  This is a swarm of pipes that extend from a coal mine in Pennsylvania to the Monteregian Hills of Quebec. Many of these kimberlite dikes are clustered around Ithaca, but the narrow swarm of kimberlites can be found crossing the state from the Pennsylvania border to Ogdensburg on the St. Lawrence River. None of the NY kimberlites are known to contain diamonds.  Diamonds have been found in the glacial deposits that were brought down from the diamond deposits of Ontario.

Kimberlite
Photo by Sub Arctic Mike

The original discovery of a kimberlite dike in New York occurred in Syracuse in 1837, but was mistaken for serpentine; it wasn’t until 1887 that rocks of this nature were properly identified at Kimberly South Africa that their true nature was discovered.  The kimberlite dikes in New York although they don’t contain diamonds because the magma came from too close to the surface of the earth do contain some other minerals that are considered gemstones providing they are large enough including olivine and spinel.

An interesting deposit of rubies and sapphires are found in the marble deposits of Orange County.  The quantity of stones that have been found in sparse, but the deposit extends into Sussex County New Jersey. These gems were found somewhere between Monroe and Southfield in Orange County. According to the report these gems were less then two carets in weight, and the deposit was sparse. This was a deposit that was worked secretly by two people for some years who took its locality to the grave with them.

When the zinc mine at Edwards was being actively mined they discovered a mass of lazulite weighing more then twenty pounds in the process of mining zinc ore.  This mass was cut up into cabochons and made into jewelry.  Lazulite is one of the components of the semi-precious stone lapis lazuli. 

A stone that is readily identified with New York State is quartz whether it is in the form of amethyst in the trap ridges along the Hudson River in the Haverstraw vicinity, at the old lead mine in Ellenville or in the Little Falls formation crossing the state from west to east where it ends at diamond cliff in Hague on Lake George.  This is the home of the world famous Herkimer Diamond that in reality is a doubly terminated quartz crystal found in cavities in the silicified dolomite of the Little Falls formation.  Various varieties of quartz (silica) gemstones are found throughout New York.  Albany County is noted for its flint and jasper that is found in sand and gravel deposits.  There was even a flint deposit overlooking the Hudson Valley that was worked by the Indians in prehistoric times for flint to make their weapons.

Gemstone Occurrences in Connecticut

Almandine garnet, Connecticut's state stone.
Photo by Rob Lavinsky

Connecticut is amply endowed with gemstones ranging from andalusite to zircons.  The stones found in Connecticut are found in a variety of terranes although the primary one is pegmatites, but they are also found in the trap ridges of the Connecticut River Valley.  Even more are found as glacial float if you want to use that method of finding gems.

Just in the town where I live, Barkhamsted there are several different verities of gemstones including almandine garnets, beryl, corundum, kyanite, quartz, soapstone, zircon and the usual rock forming minerals.

Connecticut is blessed with many gem bearing pegmatites one series starts in Glastonbury and Crosses the Connecticut River at Middletown and extends almost to the coast of Long Island Sound.  The other belt of pegmatites extends diagonally across the western portion of the state from Hartland to Ridgefield.  Both of these belts contain many of the gemstones found in LCT type pegmatites.  Many of them are Dana localities and one of them, the Maryall Mine is considered to be one of the most important gemstone mines in the United States.  This mine is famous worldwide for its crystals of aquamarine and golden beryl.  It also contains some bright red garnets that can weigh up to several pounds.

Beryl crystals on Feldspar.
Photo by Rob Lavinsky

The trap ridges of the Connecticut River Valley and in Woodbury contain specimens of amethyst, datolite and prehnite that are found in vugs in the trap rock.  Some of these occurrences are also Dana localities. Many of them are also world famous.  Connecticut has over 600 recognized mines that are now abandoned scattered mainly across the western half of the state, but there is still plenty of gems to be found in eastern Connecticut.

Microlite-Elbalite and Lepidolite found in the pegmatites of Connecticut.
Photo by Rob Lavinsky

The beaches in eastern Connecticut are littered with moonstones that have eroded from the granite eastern Connecticut is known for.  Another stop you should make is the Connecticut Mining Museum that is part of the Sloan-Stanley Museum in Kent where many of the minerals found in Connecticut are displayed along with lots of information about the mining industry that used to be found in the state.  According to many authorities modern mining was developed in Connecticut then spread out all over the world.

Gemstone Occurrences in Rhode Island

This is an amethyst scepter like those found at Diamond Hill
Photo by Rob Lavinsky

There is a greenish mica schist found in Warwick that has embedded in it red almandine garnets. In East Warwick you can find purple almandine garnets embedded in a silvery mica schist. Jamestown has staurolite crystals embedded in phyllite. Garnet-staurolite bearing schist can also be found under the Jamestown Bridge,

A discovery of beryl was recently reported in the magazine Rock and Gem on the western side of Narragansett Bay in the vicinity of the Jamestown Bridge in the pegmatites found in the area. It was also found at the end of the Cliffwalk in Newport.  The largest crystal found in Newport was about 6 cm in length and 1 cm in diameter along with several other smaller specimens.

There have been moonstones reported on the beaches of Rhode Island that continue into eastern Connecticut.  One of the beaches in Rhode Island is actually called Moonstone Beach.

The state stone is bowenite a variety of serpentine of the sub variety antigorite that has often been classified as a semi-precious stone that is cut into polished slabs or cabochons.  The Maoris of New Zealand have also used bowenite to make axes.  This stone is associated with limestone and was originally found in Lincoln.

Amethyst has been from several localities in Rhode Island notably however from Diamond Hill in Ashaway, Hopkinton in Washington County.  This mineral is accompanied by several other varieties of quarts.  At times the amethyst from this locality is found as rare scepter specimens.

A variety of Jasper veined with chalcedony is found in several localities in Cumberland, Providence County.  Chalcedony has also been found at the Conklin Quarry in Lincoln and another locality is Diamond Hill in Ashaway.  

Gemstone Occurrences in Massachusetts

This is rhodonite of the type found in the Betts Mine it is also the state gem of Massachusetts
Photo by Phillipe Giabbanelli

Massachusetts is another state that is sparsely supplied with gemstones although there are some exceptions one of which is the Betts Mine in West Cummington that produced some very fine rhodonite for years.  The rhodonite accompanied by rhodochrosite is respectively manganese silicate and carbonate both are cut as gems.  Rhodonite is also the state gem of Massachusetts. 

Rhodonite was first discovered as float from the glacier during the 19^th century where the American Gem Company part of Tiffany & Co., became aware of the float.  They used the rhodonite for making knife and umbrella handles as well as dinnerware.  There is a place setting on display in the American Museum of Natural History in New York City made from rhodonite from Massachusetts.  The deposit that is at the contact between the Hawley and Savoy schists was developed by Anson Betts who mined the rhodonite for use as flux on welding rods and later mined it as a gemstone.  The mine was bulldozed shut after the death of Anson Betts and is now closed to collecting.  Rhodonite is still available from the site in the form of mine tailings where it is still collected.

Down the road from the rhodonite deposit to the east is the small village of Lithia in the town of Goshen where the mineral called spodumene was mined in the late 1800s.  This is an area of LCT pegmatites where the first colored tourmaline was discovered in a quarry in the town of Chesterfield in the early 1800s.  Goshen is the Dana type locality for the variety of beryl termed “Goshenite.” a colorless variety of beryl.  The same pegmatites also produce blue tourmaline, petalite and pollucite a cesium aluminum silicate mineral. Beryl has also been found in a pegmatite at Royalston in Worcester  County.

This is margarite a rare form of mica on emery from the mines at Chester, MA.  Emery is the black mineral that the margarite is hosted upon.  Margarite is a brittle form of aluminum rich mica associated with emery deposits.
Photo by Rob Lavinsky

The Chester Abrasives Co. in Chester used to produce emery from the mines west of Chester in about the area where the power lines cross US RT. 20 in a series of mines running north to south for a total of five mines.  In the historical society of Chester there are four apothecary jars containing several pounds of ruby and sapphire crystals that were actually mined in the Cowee Valley corundum mines in Franklin NC operated by the Chester Abrasive Co. in the late 1900s.  Although not mined in Chester they are at least worth a good look.  There is an exposure of gem grade serpentine on the Middlefield Rd. on the right just as you are climbing out of the Westfield River Valley.

The Connecticut Valley offers a completely different suite of gemstones.  The first place to look is in the gravels of the lower Deerfield River because it contains agates that have washed from the nearby trap ridges.  The trap ridges themselves offer good deposits of amethyst and zeolites.  Lanes Quarry in Westfield is the Dana Locality for datolite, however Lanes Quarry is closed to collectors as it is an active quarry.

Datolite from Lanes Quarry in Westfield
Photo by Rob Lavinsky

Garnets are found throughout the state wherever there are crystalline rocks although many of them are rotated making them useless as gemstones.  Most of them are almandine garnets although in the Betts Mine that contains the manganese minerals rhodonite and rhodochrosite there are a rare variety of orange colored garnets called spessartine that are gemmy and suitable for faceting.  Many of these garnets tend to be small, but there are some exceptions weighing several carets.

Spessartine garnets of the type found at the Betts Mine in Plainfield MA where they occur with quartz.
Photo by Rob Lavinsky

A good source about rocks and gems in the state can be found at the many gem and mineral stores dotting the landscape.  Most of these stores stock guidebooks that give specific localities for collection, and many of them are up-to-date,

Gemstone Occurrences in Vermont

Grossular Garnet found at Eden Mills Vermont is the State Gemstone.
Photo by Rob Lavinsky

Vermont is a beautiful state, but when Mother Nature was handing out gemstones she passed over Vermont handing out very few of these stones. Finding any quality gemstones in the state is more then a little like looking for a needle in a haystack.

The Vermont Gemstone is the grossular garnet that is found at Eden Mills from the Belvidere Mine.  This mine has produced some of the best specimens of this kind of garnet that have ever been found. 

There are no extensive pegmatite deposits in Vermont although beryl crystals are found with albite in Fairfax.  Beryl crystals are found penetrating the albite at this locality.  Lake Willoughby in Orleans County is one of the few places in Vermont that does have gem bearing pegmatites.

Blue Quartz is contained in the Precambrian gneiss found in road cuts along Rt. 9 east of Bennington as you are climbing out of the valley. Smoky quartz crystals are found in the vicinity of Lowell.

Corundum has been found on Granite Hill in Cuttingsville and Shrewsbury in Rutland County Vermont according to several authorities.

Jasper is found along the East Coast of Lake Champlain in the town of Colchester.  In this locality these is a ledge sticking up in the center of a farmer’s field of jasper.  It is also found as glacial float in some of the sand & gravel deposits in the area.

Some of the marble and metamorphosed Dolostone found in western Vermont in a strip extending for the length of the state affords some fine cutting material for cabochons.

Serpentine is found in several deposits along the eastern flank of the Green Mountains extending southwards from the Canadian Border to the Massachusetts Border.  Associated with the serpentine are deposits of asbestos, soapstone and talc.  One of the best places to find soapstone is in the town of Chester.

Gold is found in the rivers and streams that drain the eastern flank of the Green Mountains.  Throughout this area placer deposits of gold have been miner for generations, and although for the most part gold is sparse one lucky miner in the 1970s reportedly hit a pocket where he recovered $2,800 worth of gold in an afternoon.

Gemstone Occurrences in Vermont

Vermont is a beautiful state, but when Mother Nature was handing out gemstones she passed over Vermont handing out very few of these stones. Finding any quality gemstones in the state is more then a little like looking for a needle in a haystack.

The Vermont Gemstone is the grossular garnet that is found at Eden Mills from the Belvidere Mine.  This mine has produced some of the best specimens of this kind of garnet that have ever been found. 

There are no extensive pegmatite deposits in Vermont although beryl crystals are found with albite in Fairfax.  Beryl crystals are found penetrating the albite at this locality.  Lake Willoughby in Orleans County is one of the few places in Vermont that does have gem bearing pegmatites.

Blue Quartz is contained in the Precambrian gneiss found in road cuts along Rt. 9 east of Bennington as you are climbing out of the valley. Smoky quartz crystals are found in the vicinity of Lowell.

Corundum has been found on Granite Hill in Cuttingsville and Shrewsbury in Rutland County Vermont according to several authorities.

Jasper is found along the East Coast of Lake Champlain in the town of Colchester.  In this locality these is a ledge sticking up in the center of a farmer’s field of jasper.  It is also found as glacial float in some of the sand & gravel deposits in the area.

Some of the marble and metamorphosed Dolostone found in western Vermont in a strip extending for the length of the state affords some fine cutting material for cabochons.

Serpentine is found in several deposits along the eastern flank of the Green Mountains extending southwards from the Canadian Border to the Massachusetts Border.  Associated with the serpentine are deposits of asbestos, soapstone and talc.  One of the best places to find soapstone is in the town of Chester.

Gold is found in the rivers and streams that drain the eastern flank of the Green Mountains.  Throughout this area placer deposits of gold have been miner for generations, and although for the most part gold is sparse one lucky miner in the 1970s reportedly hit a pocket where he recovered $2,800 worth of gold in an afternoon.

Gemstone Occurrences in New Hampshire

Aquamarine crystals (beryl) that are like those found in pegmatite pockets in New Hampshire
Photo by Gery Perant

Amethyst is found throughout New Hampshire with locations listed for practically every county in the state as well as on the Isle of Shoals off the coast from Portsmouth.  It is listed in practically all the granite quarries found throughout the State.  Amethyst is a type of quartz having a purple coloring.  The stone was originally classified as a precious stone and was numbered among the crown jewels of France.

Beryl is the official mineral of New Hampshire that is found in the many pegmatites of western New Hampshire and other granitic rocks across the state where it is found as aquamarine and golden beryl.  On a fieldtrip held in conjunction with the Gilsum Mineral Show the author was also present when emerald crystals were found in biotite schist. These were emeralds that comparable in quality with those found at Crabtree Corners, North Carolina.

Amethyst crystals like those found in New Hampshire.
Photo by Rob Lavinsky

Corundum is a mineral that often occurs in granitic pegmatites as well as pellitic limestones although we were unable to find any site specific localities in New Hampshire it is apt to be found in any silica poor rocks.  There have been reports of rubies having been found in the state.  These reports are from the Moosilauke Lake region where it is found in marble.  Corundum is often associated with kyanite in schists. Other places in the state where corundum has been found include the Belknap Mountains and the area around Wakefield. 

Fluorite is calcium fluoride used primarily as flux in the steel making process that although it is too soft to be used as a regular gem is nonetheless faceted, cut into cabochons and used as a decorative stone under circumstances where it will not be subjected to wear and rough handling.  Fluorite can be found in southwestern New Hampshire around the town of Westmoreland where it was mined at one time for flux.  Today these old mines are a collector’s paradise with beautifully colored fluorite as well as several other minerals.

Since 1985 smoky quartz has been the state gemstone of New Hampshire where it is found throughout the state.  One of the best localities for collecting smoky quartz is located in the national forest just outside Conway where it is found in rotted granite in some aggregate quarries. 

The mineralogy collection at Harvard University contains over 200 specimens of topaz crystals that were collected on South Baldface Mountain in Chatham.  Topaz is a common rock forming mineral that is often mistaken for quartz.

Tourmaline is found in the many pegmatites of New Hampshire, and it was just over the border in Maine where the fabulous find was made at Newry Maine in 1972.  Pegmatites are found in many areas of the state, but are especially prevalent in Carroll and Grafton counties.  There is a feature called the Bronson Hill Anticlinorium that is thought to be the ancient island arc that is associated with the Taconic Orogeny of Western New England.

The best place to get information about gems and gemstones is from one of the many rock shops found throughout New Hampshire.

Gemstone Occurrences in Maine

An example of carving grade tourmaline from the Dunton Quarry strike in Newry Maine.
Photo by Rob Lavinsky

There are many gemstones found in the pegmatites of Oxford County in western Maine including beryl, topaz and tourmaline, Maine’s state gemstone.  This is almost enough to make you cry, one writer describes the destruction of 225,000 carets of chrysoberyl in a blast in a pegmatite near Paris, Maine, the same writer also describes twinned crystals of this mineral weighing up to 120,000 carets that have come from the same locality.

The oldest gem mine in the United States is located on Mt. Mica an LCT Class of pegmatite in Paris the same quarry is also the second oldest known deposit of elbaite in the United States after its initial discovery at the Clarks Ledge Quarry in Chesterfield, Massachusetts. Elbaite is another name for colored tourmaline.  The Mt. Mica deposit has several other firsts including the discovery of the first rose quartz in the world.

Morganite beryl another gemstone found in Maine
Photo by Gery Parent

In the fall of 1972 a monstrous tourmaline pocket was found in the Dunton Quarry of Newry on a spur of Plumbago Mountain.  This pocket is legendary in the annals of gem mining as it yielded more then $8,000,000 worth of gem and specimen quality tourmaline.  The haul was so large the discoverers had to buy an abandoned bank building in Mexico where they stored the gem grade tourmaline in the bank vault and the specimen grade in the basement.

So important are the pegmatites of western Maine that there is a weeklong event held at the Poland Mining Camps called the Maine Pegmatite Workshop every year for the past ten years. In 2011 the workshop started on May 28. Further information about the Pegmatite Workshop can be found on their webpage.

A crystal of clear topaz
Photo by Rob Lavinsky

Tourmaline isn’t the only gemstone found in Maine there are many others.  One of these was the deposits of amethyst that came from Deer Hill in Stow that is now closed to the public.  Here the amethyst came from three quarries the Deer Hill Mine, the Nevel Mine and the Intergalactic (Eastman Ledge) Mine.  The Intergalactic Mining Company is presently mining for blue aquamarine crystals.  Aquamarine is another gemstone that is often found with tourmaline in pegmatites.

Topaz is another gemstone found in Maine with the deposits of Stoneham containing the only sizable crystals in the state.  This is a stone that is often confused with clear quartz, but glistens more when struck by sunlight.  Clear and smoky quartz crystals often accompany topaz in the states many pegmatites.  Transparent smoky quartz is often called smoky topaz or Spanish topaz if it has been heated assuming a yellowish tint.

A Maine amethyst point from Deer Hill in Stow
Photo by Rob Lavinsky

Along the banks of the St. Croix River in Perry one can find agates that have weathered from the basalt flows that are also along the river.  These basalt flows also contain native copper that was observed by the author on a collecting trip to Perry in the 1960s.  Agates are hard to see unless they have been wet by water that really makes them stand out.  The deposit also contains chalcedony and jasper.

An agate like those found at Perry along the St. Croix River
Photo by Hannes Grobe

Jasper can also be found on Jasper Beach in the district of Machias that is called Starboard along the coast right below the radar station.  The beach here is a shingle beach of rhyolite with some jasper mixed with the rhyolite stones.

There have been reports of black jade coming from the river that flows through Farmington.  If this is true it is one of the few reports of jade on the east coast.  The link for the Farmington locality also contains an extensive list of Maine minerals and their localities.

Literally there are hundreds of localities in Maine where gemstones can be found the best advice is to buy a book about the many collecting localities in the state from one of the local rock shops.

Gem Bearing Cavities in Pegmatites

Cavities in rock.
Photo by Ustill

Just about any kind of rock can have cavities some pegmatites are particularly prone to this feature, and it is in these cavities that most gemstones are found. There are many causes of these cavities, but one of the most common is a gas bubble. In many pegmatites cavities can also be formed by a lack of sufficient magma to completely fill the space the pegmatite is occupying.

Cavities are found in pegmatites generally come from one source that is the lack of sufficient material to fill the pegmatites solid. Some of these cavities are quite small and are measured in millimeters. Larger ones are measured in centimeters, and still larger ones are measured in meters. The latter are the most uncommon form of cavity. When they are found in the process of gem mining they often become legendary containing several million dollars worth of gemstones.

Rubbilite tourmaline crystals found in a gem bearing cavity in pegmatite.
Photo by Rob Lavinsky

A cavity that has not been affected by Earth movements grows a fine crop of gem crystals that are attached to the matrix. These crystals are often covered with a fine layer of clay that forms from the last of the mineral bearing solution in the cavity. One might say from the leftovers. Less than one in ten of all the cavities that are found are of this variety. Originally these pegmatites were under considerable pressure as they were formed several miles beneath the surface of the earth. As they were brought up to the surface of the earth by the process of erosion they lost this initial pressurization through the process of a diffusion of their internal gases through the pores in the rock.

Beryl, Cassiterite and Muscovite from a pegmatite cavity.
Photo by Rob Lavinsky

It's the same cavity is fractured by Earth movements so that it loses its internal pressurization crystals that it contains are shattered, and the fragments fall to the bottom of the cavity. Another feature of this type of cavity that is now open for the atmosphere allowing it to be filled with clay that accumulates in the cavity from sources found on the surface of the earth.

Finally, some cavities that remains sealed from the atmosphere are visited by a later type of mineral bearing water that causes the gem crystals inside him to be etched away partially or wholly.

These cavities are known by many names including cavity, geode, pocket and vug.

How most Colored Gems are Mined

Artesinal gem miners in Africa using traditional mining techniques.
Photo by Laura Latrigue 

Most people think that a mine is some kind of huge excavation in the ground or a series of deep tunnels that are served by the use of large equipment including 400 ton haul trucks to bring the ore to the mill. This is hardly the kind of mine describing most colored stone operations these are artisnal in nature and have hardly changed since biblical times.

The miners use as their primary tool a conical device that resembles a gold pan with a gently sloping bottom that ends in a point. The same device is also covered with small holes following easy entrance of water. This device has many names, but at least in one country where it is still used it is called the “Batik.”

An example of a rough lavender sapphire from an alluvial deposit.
Photo by Rob Lavinsky 

In use this device is filled with gem bearing gravel and then is pumped up and down in the water allowing heavier minerals to settle to the bottom of the batik. It is necessary to classify the minerals by size before being placed into the batik. Another common name for this device is the jig it because of that takes place while it is in use.

The mining takes place in the sand and gravel deposits found in association with some local stream. The gemstone deposits are usually found in close proximity to the bedrock even as gold is because most of the minerals that are classified as gemstones have a specific gravity ranging from 4 to 5 that the jigging action causes these stones to migrate to the bottom of the batik.

Sometimes the miners excavated to the gravel for 20 or more feet to reach the level of the richest deposits. Although gemstones are usually found in country that is underlain by a special type of gneiss containing the mineral cordierite like that found in the gem bearing areas of Madagascar and Sri Lanka. There is even a deposit of this special gneiss that is found in Connecticut in a narrow band extending northwards from Guilford. It is also found throughout areas that have been highly metamorphosed. It doesn't take this kind of rock to produce gemstones because they are like gold; they are where you find them.

In use once the batik has been jig up and down in the water long enough for the gemstones to settle to the bottom its top is covered with a plank and the batik is turned upside down on the plank so the gemstones are now all on top of the heap. They are then inspected for gemstones that are recovered, and the balance of the stones in the batik is discarded.

Using this method more than 50% of the colored stones that are in the jewelry trade are recovered using this method. You can make a pretty effective batik from a colander that you can buy in a kitchen supply store that is just about as effective as a batik. The most important thing that you can easily learn is what the various gemstone rough looks like.

In these times where gold is looked upon as a new measure of value it should be remembered that as far as concentrated wealth goes there is nothing that can beat gems for a very high value. Some of these gemstones are worth more than $20,000 per carat, and a caret only weighs 1/5 of a gram.

The LCT Pegmatite and the Production of Gemstones

Beryl crystals on schorl from a vug in a pegmatite
Photo by Rob Laninsky

As can be seen from the list if gemstones found in pegmatites most of them come from LCT pegmatites.  The major components of these pegmatites beside the usual feldspar, quartz and mica are Lithium, Cesium and Tantalum hence LCT pegmatites.  The three primary pegmatites grade into each other according to depth and pressure.  The simple pegmatite is the closest to the body of intrusive granite having the highest pressure and temperature.  As the temperature and pressure decreases the simple pegmatite grades into the LCT pegmatite, and finally grades into a NYF pegmatite.  The NYF pegmatite contains the elements neodymium, yttrium and fluorine hence its name NYF.

The simple pegmatite does not contain miarolitic cavities usually, but as the pegmatite grades into the LCT phase the cavities become more prevalent, and in a NYF pegmatite they are quite common.  It is in these cavities where the gems form usually as flawless crystals under the right conditions.  It is estimated that only about 10% of these cavities produce flawless gems.  These cavities are usually termed “vugs” when they are inside a pegmatite.

Topaz crystal on quartz from a pegmatite
Photo by Rob Lavinsky

If the vug is cracked or broken by earth movements it will lose its pressure causing the crystals to shatter so they are found as fragments on the floor of the vug.  Another common feature of vugs is allowing more hydrothermal fluid to enter causing this additional fluid to etch the minerals present, and sometimes completely removing them.  A late stage in the formation of a vug is the production of clay that coats the crystals with a thin layer.  Unless the integrity of the vug has been breached this clay layer is composed of leftovers from the process of pegmatization. If the vug has been breached the clay is clay that has been washed into the pegmatite from surface sources.  In either case the clay is usually kaolin.

Red tourmaline crystals with quartz and lipidolite on calcite
Photo by Rob Lavinsky

Even though the gem forming minerals are located in vugs in the center zone of the pegmatite the same minerals forming the pegmatite are found locked within the body of the pegmatite in this case the stones contain so many imperfections and fractures to render them useless as gems for the most part.  These minerals can include many of those used as gems, but in this case they can be used as ores for several important metals such as lithium, cesium and niobium and tantalum.

Most of the LCT pegmatites display zoning with each zone carrying a specific suite of minerals.  Some of these pegmatites can have as many as seven different zones. The center zone is composed mainly of quartz, and is also the zone containing the vugs.

The gem containing vugs can range in size from a few centimeters to the size of a large room.  Some of these large vugs can contain several million dollars worth of gemstones as some of the monsters in the Pala district of southern California or the monster found by two hunters in Newry, Maine that yielded $8 million worth of gem grade tourmaline.  Other fabulous finds of pegmatite gems are found all over the world.  Some of the most prominent have been found recently in Afghanistan and Pakistan that have produced many valuable gemstones.  

A list of the Gemstones found in Pegmatites

Triphylite found in a cavity in pegmatite in Newry, Maine. This is a rare gemstone
Photo by Rob Lavinsky

The most common gemstones found in pegmatites are beryl, topaz and tourmaline according to the experts there are over 100 other minerals that are found in pegmatites that are used as gemstones. The following list is of some of the many other gemstones that are found in pegmatites. Finding all the gemstones listed here will probably keep you busy for the rest of your life.

The other gemstones found in pegmatites

Name              Color               Mineral           Abundance      Pegmatite Type

achroite            c                      tourmaline        rare                  LCT

albite                g                      feldspar            common           NYF & LCT

amazonite         g                      feldspar            common           NYF > LCT    

amblygonite      y – c                amblygonite      rare                  LCT

aquamarine       b-g                  beryl                 common           LCT

beryllonite         c, pl & y         beryllonite         very rare           LCT

brazilianite        y-g                  brazilianite         very rare           LCT

chrysoberyl       g – y               chrysoberyl        very rare           LCT

danburite          c, y                  danburite            rare                  LCT

elbaite              c, p, g, b           tourmaline         common           LCT

euclase             b – g                euclase               very rare           LCT

fluoapatite         b, p, pur, g      apatite                common           NYF < LCT

goshenite          c                      beryl                  common           NYF < LCT    

hambergite        c, pl, y             hambergite        very rare           NYF & LCT

heliodor            y                      beryl                  rare                  NYF & LCT

hiddenite           g – y                 spodumene       very rare           LCT

hydroxlherderite  c, pl, y, b       hydroxyl “         very rare           LCT

indicolite           bl                     tourmaline         common           LCT

jeremejevite      b                      jeremejevite        very rare          LCT

kunzite              p                      beryl                   rare                 LCT

lazulite              b                      lazulite                rare                 LCT

lepidolite           pur – p             mica                 common           LCT

liddicoatite        p – r                 tourmaline        very rare           LCT

londonite          y – c                 londonite          very rare           LCT

manganotantalite  r                    m-tantalite        very rare           LCT

microlite           y, c-br              microlite            rare                  LCT

montebrasite     c, pl, p-y          montebrasite      rare                  LCT

morganite         p – o                beryl                  rare                  LCT

oligoclase         c                      plagioclase        common           NYF & LCT

petalite             c                      petalite               rare                  LCT

pezzottaite        r                       beryl                 very rare           LCT

phenakite          c, p, y               phenekite          rare                  LCT

pollucite            c                      Pollucite           rare                  LCT

quartz               c, p, sm, pur     quartz               common          NYF & LCT

rhodozite          y-c                   rhodozite          very rare           LCT

rossmanite        p-r, c, g            tourmaline        very rare           LCT

rubelite p-c                               tourmailine       very rare           LCT

sanidine            y, c                   feldspar           common           LCT

simpsonite        y, o                  simpsonite        very rare           LCT

spessertine        o                      garnet               common           LCT

spodumene       c-g                   spodumene       common           LCT

stibiotantalite     y                     stibiotantalite     very rare          LCT

topaz                b-c-p                topaz                 common          NYF & LCT

triplite               r, br, p             triplite               common          NYF

triphyllite           b-g                 triphyllite           rare                  LCT

verdelite           g                      tourmaline         rare                  LCT

zircon               c, p, br, g         zircon                common          NYF & LCT

Abbreviations: c, colorless; g, green; y, yellow; b, blue; p, pink; r, red; o, orange;

br, brown; pur, purple; sm, smokey; pl, pale.

After V.B. Simmons Gem bearing Pegmatites in Geology of Gem Deposits, Editor Lee A. Groat, Yukon Geological Survey

This list is incomplete, but they are always finding new minerals in pegmatites and some of them are gemstones. Some of them are so rare that only one example is known. The hunt goes on!