sedimentary (host) / mineral (crystals)
Herkimer Diamond in Matrix
Quartz (SiO2) in Dolostone (CaMg(CO3)2)
Hardness: 7.5 (Quartz) vs 3.5 (Dolostone); Color: Clear to smoky crystals in gray/tan rock; Luster: Vitreous/glassy; Structure: Doubly-terminated hexagonal crystals; Cleavage: None (conchoidal fracture).
- Hardness
- 7
- Color
- Clear to smoky crystals in gray/tan rock
- Luster
- Vitreous/glassy
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Physical properties
Hardness: 7.5 (Quartz) vs 3.5 (Dolostone); Color: Clear to smoky crystals in gray/tan rock; Luster: Vitreous/glassy; Structure: Doubly-terminated hexagonal crystals; Cleavage: None (conchoidal fracture).
Formation & geological history
Formed approximately 500 million years ago during the Cambrian Period in the sea floor. Crystals grew within small vugs (cavities) in the Little Falls Dolostone through hydrothermal deposition as the rock underwent compression and heating.
Uses & applications
Primarily for mineral collecting, jewelry (as 'raw diamonds'), and metaphysical purposes. The host rock is sometimes used as crushed stone/aggregate in construction.
Geological facts
Despite the name, they are not diamonds but exceptionally clear double-terminated quartz. They are unique to Herkimer County, New York, and surrounding areas. They are more structurally hard than common quartz, measuring 7.5 on the Mohs scale instead of 7.
Field identification & locations
Identify by looking for glassy, 18-faceted crystals embedded in a dark, crumbly, or gray limestone-like rock. Found mainly in Herkimer County, NY, and Middleville, NY. Collectors use hammers and chisels to 'bank' or crack the dolostone layers.
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