Emerald

Emerald
Emerald
	Emerald crystal from Muzo, Colombia
General
Category	Beryl variety
Formula; (repeating unit)	Be3Al2(SiO3)6
Crystal system	Hexagonal (6/m 2/m 2/m) Space group: P6/mсc
Space group	(6/m 2/m 2/m) – dihexagonal dipyramidal
Unit cell	a = 9.21 Å, c = 9.19 Å; Z = 2
Identification
Formula mass	537.50
Color	Bluish green to green
Crystal habit	Massive to well Crystalline
Cleavage	Imperfect on the [0001]
Fracture	Conchoidal
Mohs scale hardness	7.5–8
Luster	Vitreous
Streak	White
Diaphaneity	Transparent to opaque
Specific gravity	Average 2.76
Optical properties	Uniaxial (−)
Refractive index	nω = 1.564–1.595,; nε = 1.568–1.602
Birefringence	δ = 0.0040–0.0070
Ultraviolet fluorescence	None (some fracture-filling materials used to improve emerald's clarity do fluoresce, but the stone itself does not)
References

Emerald is a gemstone and a variety of the mineral beryl (Be₃Al₂(SiO₃)₆) colored green by trace amounts of chromium or sometimes vanadium.^[2] Beryl has a hardness of 7.5–8 on the Mohs scale.^[2] Most emeralds have much material trapped inside during the gem's formation,^[3] so their toughness (resistance to breakage) is classified as generally poor. Emerald is a cyclosilicate.

^ Cite error: The named reference Mindat was invoked but never defined (see the help page).
^ ^a ^b Hurlbut, Cornelius S. Jr., and Kammerling, Robert C. (1991). Gemology, John Wiley & Sons, New York, p. 203, ISBN 0-471-52667-3.
^ "Emerald Quality Factors". Gemological Institute of America. Archived from the original on 2 November 2016. Retrieved 1 November 2016.