Herkimer diamond is the trade name for double-terminated quartz crystals from Herkimer County, New York, and adjacent counties. The crystals are not diamond, despite the name, but they have been a recognised North American gemological speciality since the nineteenth century, and the trade name is sufficiently established that it is generally accepted with disclosure of the actual species.

Geological setting

Herkimer diamonds occur in vugs (small cavities) in the Little Falls Dolostone, a Cambrian-age sedimentary formation exposed in central New York State. The crystals form from silica-rich groundwater circulating through the dolostone, with crystallisation in the open vugs unrestricted on either end. The result is the characteristic double-terminated form: both ends of the crystal display the natural pyramidal point, rather than the single-terminated form that develops when one end of the crystal is attached to a substrate.

The crystals are typically water-clear with high transparency, although some specimens contain inclusions of bituminous material (from the carbon-bearing dolostone), liquid-filled cavities, or rare anhydrite inclusions. The size range is generally small to medium, with most specimens under three centimetres in length, although larger crystals have been recovered.

Properties

The crystals are pure quartz with the standard quartz properties: hardness 7 on the Mohs scale, refractive index 1.544 to 1.553, specific gravity 2.65, hexagonal trigonal crystal system. The doubly terminated form is the principal distinguishing feature; the high transparency and the natural facet quality of the pyramidal terminations distinguish Herkimer material from quartz from many other localities.

The crystals do not require cutting to display the characteristic appearance. The natural facets are typically of high quality, with sharp edges and well-formed terminations. Faceted Herkimer material is occasionally produced, but the unfaceted natural crystal is the principal form in which the material reaches the trade.

Sources and production

The principal collecting localities are around the towns of Middleville, Newport, and Salisbury in Herkimer and Montgomery counties of central New York. Several commercial mines and pay-to-dig sites operate in the area, producing material both for direct sale and for the wholesale jewellery and metaphysical-stone markets. The Herkimer Diamond Mines KOA, the Crystal Grove Diamond Mine, and the Ace of Diamonds Mine are among the operations. Production volumes are modest by comparison with major Brazilian or Madagascar quartz operations, but the unique character of the material supports a stable market.

Similar double-terminated quartz crystals occur in other localities, including parts of China, Pakistan, Mexico, and several other US states. These materials are sometimes marketed using the Herkimer designation, although the strict trade convention is that only material from the Herkimer area is properly called Herkimer diamond.

Position in the trade

Herkimer diamond is sold across a wide range of markets: as collectible mineral specimens, as jewellery components (often in their natural form, drilled or wire-wrapped), and in the metaphysical or new-age channel where the material has acquired specific associations. Prices range from a few dollars per gram for ordinary material to several dollars per carat for high-clarity faceted stones and specimen-quality natural crystals.

The trade has generally accepted the term Herkimer diamond on the basis of long-standing usage, with the understanding that the material is quartz rather than diamond. The CIBJO Coloured Stone Book and the FTC Jewelry Guides allow the use of locality-based names for varieties provided that the species is also disclosed. The trade convention is that the term Herkimer diamond should be accompanied by clear identification as quartz at the point of sale, particularly to consumers unfamiliar with the trade nomenclature.