How Diamond Crystals Grow

A diamond crystal from Crater of Diamonds State Park, Arkansas
Photo by Rob Lavinsky

Diamonds don’t grow overnight because in nature their growth proceeds one atom of carbon at a time with four carbon atoms bound together by chemical bonds.  The natural way that this bonding occurs will normally create a crystal that is an octahedron in shape.  This shape however can be altered by several different parameters imposed by the space allocated to the growth of the crystal in the media in which it is formed.  In the process of growing diamonds are apt to include small crystals of the surrounding material into their crystal shape.  These inclusions are literally windows into the mantle of the earth that are brought to the surface when the diamond is carried to the surface of the earth by a kimberlite or lamproite.

Diamonds are nothing more complicated then a crystal of pure carbon, and are in fact an allotropic form of an element that can have other crystalline forms either amorphous as lampblack or as graphite from which we make pencils.  There is another rarer amorphic form of carbon termed ‘lonsdalite’ that is somewhere between graphite and diamond in physical structure.  Lonsdalite is not a variety of diamond but is instead a different material.  In lonsdalite there is a repeating networking of carbon atoms that all point in the same direction rather then alternating back and forth as in a diamond.  Its crystals instead of forming cubes or tetrahedra as the diamond does are instead hexagonal.  Although it is often found with diamonds it is considered to be an allotropic form of carbon formed as the result of shock from a large meteorite.

From work performed on some of the diamonds from the Ekati diamond mine in the Northwest Territories of Canada it would appear that the growth of diamond crystals begin with a small fragment of graphite upon which the diamond crystals are deposited over time.  A matter of controversy is the actual speed in which diamond crystals are formed.  Experiments in Japan with manmade diamond crystals suggest that it is possible to grow a ten caret crystal in several hundred hours.  This of course is under ideal conditions and in nature would probably take much longer as the diamond crystal would have to scavenge its carbon atoms from the surrounding rocks.  Carbon is a rare element in the earth’s rocks.

Most of the carbon available to create a diamond has been postulated to come from oceanic crust that has been subducted beneath a continental crust as deceased life forms of coral reefs with the necessary temperature to cause their disassociation into the components making up the coral reef.  This would be carbon and the carbonate radical.  The calcium or magnesium from this reaction would be incorporated into the earth’s mantle.  Whatever free carbon remains from this reaction would be available at this temperature and pressure to form diamond crystals.

As the diamond crystal grows it is also scavenging other elements that are incorporated into the crystal as crystal defects.  The most common of these elements are nitrogen and iron.  These elements cause the diamond crystal to become colored with the coloration being dependent upon the amount of these elements.

References:

Harlow, George E., The Nature of Diamonds, American Museum of Natural History, © 1995

The Nature of Diamonds, http://www.amnh.org/exhibitions/diamonds/

Mineralogical Association of Canada, Editor Lee A. Groat, Geology Of Gem Deposits, Short Course Series, Volume 37, Yukon Geological Survey, © 2007, Yellowknife, Northwest Territories
Gem Deposits, http://amonline.net.au/geoscience/earth/gem.htm
Geology of Gem Deposits, Mineralogical Association of Canada, Editor Lee A. Groat, http://www.mineralogicalassociation.ca/doc/promo_SC37.pdf  Volume 37 © 2007

Apatite as a gemstone

A crystal of gemmy apatite on a matrix of quartz.
Photo by Rob Lavinsky

Apatite is a whole family of minerals some of them are gems, but the place where you are most familiar with is your teeth and skeleton that are composed of hydroxalapatite.  Basically apatite is composed of calcium phosphate with extra ions added to its crystalline structure to cause it to be known as several related names: flourapatite, chlorapatite, bromapatite and in the case of your bones the OH ion. Apatite is used as the defining mineral for number five on the Moh’s hardness scale. Hydroxyapatite is a relatively rare form of the mineral are mostly groups are absent or contain many carbonate and acid phosphate substitutes that form a large part of bone material.

A twinned crystal of Apatite
Photo by Rob Lavinsky

Flourapatite is much more resistant to acid attack than hydroxyapatite it is for this reason that fluorine is added to your drinking water and can also be found in your toothpaste. In the case of the fluoridated water it allows the interchange of fluoride ions that substitute for hydroxyapatite ions. If you get too much fluoride into your system it can result in dental or skeletal flouresis.

Fission tracks are used by geochemists and geophysicists for determining the thermal history of orogenic belts and settlements in sedimentary basins. This is because as a trace element uranium and thorium are found in crystalline apatite. The same phenomenon is also used in paleontology to establish the dates of prehistoric wildfires.

A faceted gemstone of apatite.
Photo by Rob Lavinsky

Apatite in phosphorite is a phosphorus rich sedimentary rock containing between 18 and 40% of phosphorus pentoxide (P₂O₅). A great deal of the phosphorite is mined in central Florida in an area that is called “Bone Valley” for use as fertilizer. It contains so much uranium that during the Cold War if supplied several thousand tons of yellowcake uranium to the Atomic Energy Commission. The piles of mine slimes resulting from mining and refining this phosphorite presents a serious environmental hazard not only from the uranium, but also from the contained fluorides. The apatite contained in phosphorite is in crypto- crystalline masses that are called colophane. Apatite is also mined from igneous rocks in the Kola Peninsula of northeast Russia as a fertilizer as well..

Clear crystals of apatite can be faceted creating an extremely handsome gem. Because of the sensitivity to heat and shock gems that are cut from apatite are never heat-treated to improve their looks. Although apatite is found all over the world most of it is small crystals disseminated through the rocks; larger crystals are quite rare. Most of the faceting grade apatite crystals are shades of blue, although some are blue-green.

A thin section of hornblende and apatite
Photo by Pitor Sosnowski

Most of this faceting material is found in metamorphosed limestone that is commonly called marble. In general the more impurities that are found in limestone the more apt it is to produce apatite when it is metamorphosed. There is a bed of metamorphosed limestone that reaches all the way from Québec to Alabama just of the West of the Precambrian core of the Appalachians. In many places this is called the Great Valley of the Appalachians but the same marble belt extends all the way to Canada. There is apatite found throughout this entire region, and others like it all over the world.

Aquamarine as a Gemstone

Aquamarine is a member of the beryl family of gemstones that is usually colored various shades of blue/green or may appear as either light green or even as dark blue stones rivaling sapphire in color.  The mineral is a beryllium aluminum cyclosilicate with the formula Be₃Al₂(SiO₃)₆.  Terminated crystals of beryl are relatively rare, but are known to occur.  The stone has a value as a gemstone, and is a common ore of the metal beryllium.  In the case of aquamarine a small amount of iron that causes crystal defects in the crystal is the cause of its color.  The mineral beryl because of impurities in the crystal does occur in several different colors including green blue, yellow, red and white.

Aquamarine in a matrix of feldspar crystals.and other minerals including schorl the black crystal in the middle of the specimen.
Photo by Rob Lavinsky

The colors of aquamarine can range from light blue like that of the sky to a deep blue of the sea.  The darker the color the higher the price some aquamarines can sell for as much as $2,000 per caret although lighter stones only fetch a few dollars per carat.  The stone itself is fascinatingly beautiful with much appeal to women all over the world because it can complement or eye color.  For creative gemstone cutters it is an inspiration because of its ability to be cut in so many designs and still keep its beauty.

A water etched crystal of aquamarine from Brazil
Photo by Rob Lavinsky

Aquamarine is epitomized by its blue color, and the deeper blue the color the more expensive the stone making it one of the most popular stones rivaling at times its more expensive cousin emerald that also belongs to the beryl family.  The stone is hard enough at Mohs hardness of 7.5 to 8 so it can be used in most types of jewelry making it almost as popular as the classic stones emerald, ruby and sapphire.  In reality aquamarine is related to the emerald, but unlike its more famous cousin it usually free of flaws.  One flaw that is sometimes seen in aquamarine is long thin tubes that sometimes lend to its beauty in the form of chatoyancy forming tiger eyes or more rarely a star stone much like star sapphire.  Both forms of the stone command very high prices.

An example of a pear shaped aquamarine faceted gem.
Photographer Unknown

The color of aquamarine is caused by ions of iron causing crystal defects in the mineral beryl that is normally clear that is called goshenite my mineralogists because its type locality is in Goshen, Massachusetts were it is found in lithium bearing pegmatites.  Aquamarine can range from a pale blue to a strongly colored sea blue.  In some aquamarines you can see a light greenish simmer that is a typical feature of the stone.  Thee most prized color however is a pure clear blue transparent stone bringing out the transparency and shine of this magnificent stone.