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Monday, September 12, 2011

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.  

Saturday, September 3, 2011

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.

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.