Material properties of diamond

Diamond
An octahedral diamond crystal in matrix
General
Category	Native Nonmetal, Mineral
Formula (repeating unit)	Carbon (C)
Crystal system	Diamond cubic (a = 3.56683 Å)
Identification
Color	Most often colorless to yellow or brown. Rarely pink, orange, green, blue, gray, or red.
Crystal habit	Octahedral, cubo-octahedral, spherical or cubic
Cleavage	Octahedral; perfect and easy
Fracture	Conchoidal
Mohs scale hardness	10
Streak	white
Diaphaneity	Clear to not
Specific gravity	3.516–3.525
Refractive index	2.417
Pleochroism	None
Fusibility	Burns above 700 °C in air.
Solubility	Resistant to acids, but dissolves irreversibly in hot steel
Major varieties
Ballas	Spherical, radial structure, cryptocrystalline, opaque black
Bort	Poorly formed, cryptocrystalline, shapeless, translucent
Carbonado	Massive, microcrystalline, opaque black

Diamond is the allotrope of carbon in which the carbon atoms are arranged in the specific type of cubic lattice called diamond cubic. Diamond is an optically isotropic crystal that is transparent to opaque. Owing to its strong covalent bonding, diamond is the hardest naturally occurring material known. Yet, due to important structural weaknesses, diamond's toughness is only fair to good. The precise tensile strength of diamond is unknown, however strength up to 60 GPa has been observed, and it could be as high as 90–225 GPa depending on the crystal orientation. The anisotropy of diamond hardness is carefully considered during diamond cutting. Diamond has a high refractive index (2.417) and moderate dispersion (0.044) properties which give cut diamonds their brilliance. Scientists classify diamonds into four main types according to the nature of crystallographic defects present. Trace impurities substitutionally replacing carbon atoms in a diamond's crystal structure, and in some cases structural defects, are responsible for the wide range of colors seen in diamond. Most diamonds are electrical insulators but extremely efficient thermal conductors. Unlike many other minerals, the specific gravity of diamond crystals (3.52) has rather small variation from diamond to diamond.