Friday, August 14, 2009

Herkimer Diamonds

Doubly terminated quartz crystals named for Herkimer County, New York
Herkimer Diamond Quartz Crystals
"Herkimer Diamonds" is the name given to the doubly terminated quartz crystals found in Herkimer County, New York and surrounding areas. Examples of these crystals are shown in the photo at right. Note that these crystals have the typical hexagonal habit of quartz, however, instead of having a termination on one end they are doubly terminated. This is a result of the crystals growing with very little or no contact with their host rock. Such doubly terminated crystals are very rare and this is part of what makes Herkimer Diamonds so popular with mineral collectors.
The host rock for Herkimer Diamonds is the Cambrian-age, Little Falls Dolostone. The Little Falls Dolostone was deposited about 500 million years ago and the Herkimer Diamonds formed in cavities within the dolostone. These cavities are frequently lined with drusy quartz crystals and often are coated with a tarry hydrocarbon (see image below).Although Herkimer County, New York is the location for which these crystals are named, similar doubly terminated quartz crystals have been found in a few other locations, including Arizona, Afghanistan, Norway, Ukraine and China. They have the same appearance but can not rightfully be called "Herkimers". The doubly terminated quartz crystals shown in the lower right photo are from a deposit in Afghanistan.
"Herkimer Diamonds" found in Afghanistan. Similar crystals are found at several locations.
Who Discovered Herkimer Diamonds?The Herkimer Diamonds of New York are not a recent discovery. The Mohawk Indians and early settlers knew about the crystals. They found them in stream sediments and plowed fields. These people were amazed with the crystals and immediately held them in high esteem.
Herkimer Diamond MinesSome of the best places to find Herkimer Diamonds today are located along New York State Route 28 in Middleville, New York. (When visiting this area it is important to remember that all land in New York either belongs to the government or is private property. Collecting minerals from government lands is illegal in New York and collecting on private property always requires permission in advance.) There are two commercial mines on New York State Route 28 at Middleville, New York. Both allow collectors to enter and prospect for a nominal fee. Both locations also rent equipment such as hammers, wedges and other small tools. They also have small exhibit areas where you can view and/or purchase specimens.
Mining for Herkimer DiamondsThe key to finding Herkimer Diamonds is a knowledge that they occur in cavities (vugs) within the Little Falls Dolostone These cavities can be smaller than a pea or several feet across. At both of the mines listed above the Little Falls Dolostone is exposed at the surface and a significant amount of broken rock is scattered across the quarry floor.
"Find and Break" ProspectingThe easy way to prospect is to find pieces of vuggy rock and break them open with a heavy hammer. If you are lucky the rock will break to reveal one or several Herkimer Diamonds within a cavity. If your visit to the mine will last just a few hours or even a single day this is a good way to spend your time.Dolomite is a very tough rock so expect to work hard. The use of safety glasses is required and wise collectors wear gloves to protect their hands. We always wear jeans or heavy long pants and a long sleeve shirt for "find and break" prospecting. Small pieces of dolomite will sometimes fly when a rock breaks and they can easily cut or bruise a person wearing short pants. The "find and break" prospecting method described above is employed by many people who visit these mines and can lead to a few good finds. The keys to success are selection of good rocks to break and not being discouraged if you break fifty rocks without finding a crystal. (See image below to know what "vuggy rock" looks like. "Scavenger" ProspectingSome visitors to the mines have been successful by simply searching the rock rubble for exposed crystals or searching the quarry floor for loose crystals. We found several really nice crystals this way and lots of tiny ones. We have also seen children find many nice crystals this way.
"Cavity" ProspectingFor finding large quanties of crystals, the most successful mining method is to break into large cavities in the quarry walls and floors using sledge hammers and wedges (power equipment is not permitted at the mines listed in this article). This method requires tools, patience, time and a knowledge of how to break an extremely durable dolostone.
Some of Bill McIlquham's diamond prospecting tools: mainly hammers and wedges
Driving wedges into a crevice to break the dolostone: multiple wedges at several points.

If you like minerals and have an opportinity to visit the Herkimer County area of New York, consider spending a day looking for Herkimer Diamonds. Be sure to wear clothes that are suitable for working outdoors. Safety glasses are required and you will be sorry if you don't wear gloves. If you need a sledge hammer or other tools you can rent them at the mine for a very small fee.

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