Herkimer diamonds are double-terminated

quartz Quartz is a hard, crystalline mineral composed of silica ( silicon dioxide). The atoms are linked in a continuous framework of SiO4 silicon-oxygen tetrahedra, with each oxygen being shared between two tetrahedra, giving an overall chemical ...

crystals discovered within exposed outcrops of

dolomite Dolomite may refer to: *Dolomite (mineral), a carbonate mineral *Dolomite (rock), also known as dolostone, a sedimentary carbonate rock *Dolomite, Alabama, United States, an unincorporated community *Dolomite, California, United States, an unincor ...

in and around

Herkimer County, New York Herkimer County is a county in the U.S. state of New York. As of the 2020 census, the population was 60,139. Its county seat is Herkimer. The county was created in 1791 north of the Mohawk River out of part of Montgomery County. It is named ...

and the

Mohawk River The Mohawk River is a U.S. Geological Survey. National Hydrography Dataset high-resolution flowline dataThe National Map accessed October 3, 2011 river in the U.S. state of New York. It is the largest tributary of the Hudson River. The Mohawk f ...

Valley. Not actually

diamond Diamond is a solid form of the element carbon with its atoms arranged in a crystal structure called diamond cubic. Another solid form of carbon known as graphite is the chemically stable form of carbon at room temperature and pressure, ...

s, the "diamond" in their name is due to both their clarity and natural faceting - crystals possess double termination points and 18 total faces (six on each point, six around the center). Because the first discovery sites were in the village of Middleville and in the city of Little Falls, respectively, the crystal is also known as a Middleville diamond or a Little Falls diamond. Herkimer diamonds became widely recognized after workmen discovered them in large quantities while cutting into the Mohawk River Valley dolomite in the late 18th century. Geologists discovered exposed dolomite in Herkimer County outcroppings and began mining there, leading to the "Herkimer diamond" moniker. Double-point quartz crystals may be found in sites around the world, but only those mined in Herkimer County can be given this name.

Process of formation

The geologic history of these crystals began about 500 million years ago in a shallow sea which was receiving sediments from the ancient

Adirondack Mountains The Adirondack Mountains (; a-də-RÄN-dak) form a massif in northeastern New York with boundaries that correspond roughly to those of Adirondack Park. They cover about 5,000 square miles (13,000 km2). The mountains form a roughly circular ...

to the north. The

calcium Calcium is a chemical element with the symbol Ca and atomic number 20. As an alkaline earth metal, calcium is a reactive metal that forms a dark oxide-nitride layer when exposed to air. Its physical and chemical properties are most similar t ...

and magnesium carbonate sediments accumulated and lithified to form the dolomite bedrock currently known as the

Little Falls Formation The Little Falls Formation or Little Falls Dolostone is a geologic formation in New York. The unit is of Cambrian The Cambrian Period ( ; sometimes symbolized Ꞓ) was the first geological period of the Paleozoic Era, and of the Phanerozoi ...

and formerly as the Little Falls Dolostone.''Herkimer Diamonds,''
MAGS Explorer, V. 2 No. 7, pp. 1-3, July 11, 2003 While buried, cavities were formed by acidic waters forming the

vug A vug, vugh, or vugg ( ) is a small- to medium-sized cavity inside rock. It may be formed through a variety of processes. Most commonly, cracks and fissures opened by tectonic activity (folding and faulting) are partially filled by quartz, ...

s in which the quartz crystals formed. While the dolomite unit is Cambrian in age, the quartz within the vugs is interpreted to have formed during the

Carboniferous Period The Carboniferous ( ) is a geologic period and system of the Paleozoic that spans 60 million years from the end of the Devonian Period million years ago ( Mya), to the beginning of the Permian Period, million years ago. The name ''Carbonifero ...

. Waxy organic material, silicon dioxide and

pyrite The mineral pyrite (), or iron pyrite, also known as fool's gold, is an iron sulfide with the chemical formula Fe S2 (iron (II) disulfide). Pyrite is the most abundant sulfide mineral. Pyrite's metallic luster and pale brass-yellow hue giv ...

(iron sulfide) was present as minor constituents of rock made of

and calcite. As sediment buried the rock and temperatures rose, crystals grew in the cavities very slowly, resulting in quartz crystals of exceptional clarity. Inclusions can be found in these crystals that provide clues to the origins of the Herkimer diamonds. Found within the inclusions are solids, liquids (salt water or petroleum), gases (most often

carbon dioxide Carbon dioxide ( chemical formula ) is a chemical compound made up of molecules that each have one carbon atom covalently double bonded to two oxygen atoms. It is found in the gas state at room temperature. In the air, carbon dioxide is trans ...

), two- and three-phase inclusions, and negative (uniaxial) crystals. A black hydrocarbon is the most common solid inclusion.

References

External links

Website for the New York State Academy of MineralogyProspecting for Herkimer DiamondsHerkimerHistory.comVideo of mining for Herkimer Diamonds with a jackhammerBook Review: Collector’s Guide to Herkimer Diamonds
{{DEFAULTSORT:Herkimer Diamond Quartz varieties

Diamond Diamond is a solid form of the element carbon with its atoms arranged in a crystal structure called diamond cubic. Another solid form of carbon known as graphite is the chemically stable form of carbon at room temperature and pressure, ...

Process of formation

See also

References

External links