Carnelian

Carnelian
General
Category	Chalcedony variety
Formula (repeating unit)	SiO2 (silicon dioxide)
Crystal system	Trigonal
Identification
Formula mass	60 g/mol
Color	Reddish, orange, brownish
Cleavage	Absent
Fracture	Uneven, splintery, conchoidal
Mohs scale hardness	6.5–7.0
Luster	Waxy to resinous
Streak	White
Diaphaneity	Translucent to opaque
Specific gravity	2.58–2.64
Optical properties	Uniaxial +
Refractive index	1.535–1.539
Birefringence	0.003–0.009
References	[1][2]

Carnelian (also spelled cornelian)^[3] is a brownish-red mineral commonly used as a semiprecious stone. Similar to carnelian is sard, which is generally harder and darker; the difference is not rigidly defined, and the two names are often used interchangeably. Both carnelian and sard are varieties of the silica mineral chalcedony colored by impurities of iron oxide. The color can vary greatly, ranging from pale orange to an intense almost-black coloration. Significant localities include Yanacodo (Peru); Ratnapura (Sri Lanka); and Thailand.^[2] It has been found in Indonesia, Brazil, India, Russia (Siberia), and Germany.^{[citation needed]} In the United States, the official State Gem of Maryland is also a variety of carnelian called Patuxent River stone.^[4]