Emerald

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Emerald
7.7777.JPG
Cut Emerald
General
CategoryBeryl variety
Formula
(repeating unit)
Be3Al2(SiO3)6
Crystal systemHexagonal (6/m 2/m 2/m) Space group: P6/mсc
Space group(6/m 2/m 2/m) – dihexagonal dipyramidal
Unit cella = 9.21 Å, c = 9.19 Å; Z = 2
Identification
Formula mass537.50
ColorGreen shades to colorless
Crystal habitMassive to well Crystalline
CleavageImperfect on the [0001]
FractureConchoidal
Mohs scale hardness7.5–8
LusterVitreous
StreakWhite
DiaphaneityTransparent to opaque
Specific gravityAverage 2.76
Optical propertiesUniaxial (−)
Refractive indexnω = 1.564–1.595,
nε = 1.568–1.602
Birefringenceδ = 0.0040–0.0070
Ultraviolet fluorescenceNone (some fracture-filling materials used to improve emerald's clarity do fluoresce, but the stone itself does not)

Emeralds are a variety of the mineral beryl Be3Al2(SiO3)6), which has small amounts of chromium or vanadium mixed in.