Beryl

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Beryl
Chemical composition Be3Al2(SiO3)6 Beryllium aluminum silicate
Crystal system Hexagonal
Habit Prismatic
Cleavage Very difficult in one direction, rarely seen
Fracture Conchoidal
Hardness 7.25-7.75
Optic nature Uniaxial -
Refractive index 1.577-1.583
(+ or - 0.017)
Birefringence 0.005-0.009
Dispersion Low, 0.014
Specific gravity 2.72
(+0.18,- 0.05)
Lustre Vitreous to resinous
Pleochroism Weak to moderate