Herkimer Diamond Quartz Crystals
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"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.
Cavity with drusy quartz, hydrocarbon and a nice diamond. Rock is 6" across. Zoom in for detail. |
 "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 Mines
Some 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. These are: Ace of Diamonds Mine and
Herkimer Diamond Mine. 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 Diamonds
The key to finding Herkimer Diamonds is a knowledge that they occur in cavities (vugs) within the
Little Falls Dolostone (see photo above). 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" Prospecting
The 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. Click the image for a closer view.)
Vuggy rock containing a nice Herkimer Diamond. Rock is about six inches across.
"Scavenger" Prospecting
Some 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" Prospecting
For 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.
Photo © Bill McIlquham. |
Driving wedges into a crevice to break the dolostone: multiple wedges at several points.
Photo © Bill McIlquham |
On a recent visit to the Ace of Diamonds Mine at Middleville, New York we met Bill McIlquham of Peterborough, Ontario. Bill was mining for Herkimers with his wife Anne,
their friend Laurie Mullett and mascot Duffy the Rockhound. They had located a large cavity and were carefully opening it. (Photos of their work shown here were kindly shared with Geology.com by Bill and fellow miners Cheryl Haberman and Alan Summer.)
Bill uses a railroad tie-puller to open a cavity.
Photo © Bill McIlquham |
 Duffy the Rockhound supervises the the dig.
Photo © Bill McIlquham |
The McIlquhams have been mining for Herkimers for about 12 years and have found many large
cavities. A key element in their success is a nice array of hammers, wedges and pry-bars. Instead of bashing
the dolomite repeatedly with a hammer to break it into tiny pieces, Bill uses a sledge hammer
and wedges to very carefully exploit existing fractures in the rock. He begins by placing one wedge in a
fracture and tapping it an inch or two deep. A second wedge is tapped into the fracture and additional wedges
are used if needed. These wedges exert forces that penetrate
into the rock and break large blocks of dolostone free. Fractures within the large dolostone blocks are then located
and exploited until the large block has been reduced to smaller pieces that can be lifted from the quarry.
Later that afternoon - finally getting into the cavity!
Photo © Bill McIlquham |
Paydirt! First look into a new cavity!
Photo © Bill McIlquham |
Anne, Bill & Duffy relax after opening a nice cavity.
Photo © Bill McIlquham |
If a collector is lucky and determined to prevail over the durable dolostone, the reward could be
breaking into a cavity. These cavities can contain a few to a few thousand nice Herkimer Diamonds that range in size from a
couple of millimeters to over twenty centimeters in size. Perfect single crystals, doubles and crystal clusters might all
be found in a single cavity.
The cavity shown above was opened by Anne and Bill. It
contained over one hundred quartz crystals in a variety of sizes, ranging from a few millimeters to several centimeters in length. A very nice prize for a day's work! Two large clusters from the cavity are shown below
Large crystal cluster from cavity. Click to enlarge.
Photo © Bill McIlquham |
 Another cluster - "The Squirrel". Click to enlarge.
Photo © Bill McIlquham |
Herkimer Diamond Specimens & Jewelry
Why hunt for Herkimer Diamonds? It's great fun and every time you break open a rock you will
look with anticipation to see if you liberated an unseen quartz crystal.
Nice Herkimer Diamonds are highly prized mineral specimens and are sought by mineral collectors worldwide. Large
numbers of Herkimer crystals are also used in jewelry because
their natural facets are both beautiful and interesting. Some people also seek Herkimer diamonds because they are thought to have
"holistic qualities".
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. If you want to obtain some nice Herkimer Diamonds but are unable to visit Herkimer to mine them yourself please visit Bill's site at HerkimerDiamonds.ca.
Contributor: Hobart King
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