Diamond

Diamond is a solid form of the element carbon with its atoms arranged in a crystal structure called diamond cubic. Another solid form of carbon known as graphite is the chemically stable form of carbon at room temperature and pressure, but diamond is metastable and converts to it at a negligible rate under those conditions. Diamond has the highest hardness and thermal conductivity of any natural material, properties that are used in major industrial applications such as cutting and polishing tools. They are also the reason that diamond anvil cells can subject materials to pressures found deep in the Earth.

Diamond
The slightly misshapen octahedral shape of this rough diamond crystal in matrix is typical of the mineral. Its lustrous faces also indicate that this crystal is from a primary deposit.
General
CategoryNative minerals
Formula
(repeating unit)
C
IMA symbolDia[1]
Strunz classification1.CB.10a
Dana classification1.3.6.1
Crystal systemCubic
Crystal classHexoctahedral (m3m)
H-M symbol: (4/m 3 2/m)
Space groupFd3m (No. 227)
Structure
Jmol (3D)Interactive image
Identification
Formula mass12.01 g/mol
ColorTypically yellow, brown, or gray to colorless. Less often blue, green, black, translucent white, pink, violet, orange, purple, and red.
Crystal habitOctahedral
TwinningSpinel law common (yielding "macle")
Cleavage111 (perfect in four directions)
FractureIrregular/Uneven
Mohs scale hardness10 (defining mineral)
LusterAdamantine
StreakColorless
DiaphaneityTransparent to subtransparent to translucent
Specific gravity3.52±0.01
Density3.5–3.53 g/cm3 3500–3530 kg/m3
Polish lusterAdamantine
Optical propertiesIsotropic
Refractive index2.418 (at 500 nm)
BirefringenceNone
PleochroismNone
Dispersion0.044
Melting pointPressure dependent
References[2][3]
Main diamond producing countries

Because the arrangement of atoms in diamond is extremely rigid, few types of impurity can contaminate it (two exceptions are boron and nitrogen). Small numbers of defects or impurities (about one per million of lattice atoms) color diamond blue (boron), yellow (nitrogen), brown (defects), green (radiation exposure), purple, pink, orange, or red. Diamond also has a very high refractive index and a relatively high optical dispersion.

Most natural diamonds have ages between 1 billion and 3.5 billion years. Most were formed at depths between 150 and 250 kilometres (93 and 155 mi) in the Earth's mantle, although a few have come from as deep as 800 kilometres (500 mi). Under high pressure and temperature, carbon-containing fluids dissolved various minerals and replaced them with diamonds. Much more recently (hundreds to tens of million years ago), they were carried to the surface in volcanic eruptions and deposited in igneous rocks known as kimberlites and lamproites.

Synthetic diamonds can be grown from high-purity carbon under high pressures and temperatures or from hydrocarbon gases by chemical vapor deposition (CVD). Imitation diamonds can also be made out of materials such as cubic zirconia and silicon carbide. Natural, synthetic and imitation diamonds are most commonly distinguished using optical techniques or thermal conductivity measurements.


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