GPFDC

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<math>{n \choose k}</math> <math>\int_{-N}^{N} e^x\, dx</math>

Diamond
Chemical composition C
Crystal system Cubic
Habit Octahedral, cubic
Cleavage Perfect (octahedral)
Fracture Conchoidal
Hardness 10
Optic nature Isotropic
Refractive index 2.417
Birefringence None
Dispersion High, 0.044
Specific gravity 3.52
Lustre Adamantine
Pleochroism None
Corundum
Chemical composition Al2O3
Crystal system Trigonal
Habit Prismatic, tabular
Cleavage None; twinned stones may show parting
Fracture Conchoidal
Hardness 9
Optic nature Uniaxial -
Refractive index 1.762-1.770
(+.009,-.005)
Birefringence 0.008-0.010
Dispersion Low, 0.018
Specific gravity 4
Lustre Vitreous to Subadamantine
Pleochroism Weak to moderate
Kyanite
Chemical composition Al[sub]2[/sub]SiO[sub]5[/sub]
Crystal system triclinic
Habit bladed
Cleavage parallel to prism face
Fracture splintery or fibrous
Hardness 7 across the crystal, 5 along the length
Optic nature trichroic
Refaction index 1.715-1.732
Birefringence 0.017
Dispersion low, 0.011
Specific gravity 3.58
Lustre vitreous
Pleochroism pronounced violet-blue, colorless, and blue