“Herkimer diamonds” share most of the physical properties of other forms of quartz. They are almost always transparent and range from colorless to smoky in color. “Herkimer diamonds” are, by definition, doubly terminated; however, they are known to occur in a wide range of crystal forms (see illustration).

The crystals contain a wide range of inclusions. Particles of solid hydrocarbon materials are the most common inclusion. They range from small eye-visible particles down to micron-size particles, which, when abundant, can impart a smoky color to the crystals. Salt water and liquid petroleum are the most common liquid inclusions. Carbon dioxide is the most common gaseous inclusion. Calcite, dolomite, pyrite, sphalerite, and quartz (often in the form of tiny “Herkimer diamonds”) are common mineral inclusions.

Physical Properties of “Herkimer Diamonds”
Chemical Classification	Silicate
Color	Colorless to smoky
Streak	Colorless (harder than the streak plate)
Luster	Vitreous
Diaphaneity	Transparent
Cleavage	None – typically breaks with a conchoidal fracture
Mohs Hardness	7
Specific Gravity	2.6 to 2.7
Diagnostic Properties	Conchoidal fracture, glassy luster, hardness, double terminations
Chemical Composition	SiO2
Crystal System	Trigonal
Uses	Sought by mineral and crystal collectors. Used to make natural crystal jewelry. Faceted for use in jewelry and as collector gems. Sold as a souvenir of Herkimer County, and collecting sites are promoted as tourist attractions. Frequently used as a talisman, amulet, or holistic gemstone.