Carbonado

Carbonado
Three carbonados from the Central African Republic
General
Category	Native minerals
Formula (repeating unit)	C
Crystal system	Isometric-hexoctahedral (cubic)
Identification
Formula mass	7001120100000000000♠12.01 u
Color	Typically black, can be grey, various shades of green and brown sometimes mottled.
Crystal habit	Polycrystalline
Fracture	Irregular torn surfaces
Mohs scale hardness	10
Luster	Adamantine
Streak	White
Specific gravity	7000352000000000000♠3.52±0.01
Density	3.5–7003353000000000000♠3.53 g/cm3
Polish luster	Adamantine
Birefringence	None
Pleochroism	None

Carbonado, commonly known as the "black diamond", is the toughest form of natural diamond. It is an impure form of polycrystalline diamond consisting of diamond, graphite, and amorphous carbon. It is found primarily in alluvial deposits in the Central African Republic and in Brazil. Its natural colour is black or dark grey, and it is more porous than other diamonds.

The characteristics of carbonado noted in this section are based mainly on the summary of Heaney et al. (2005), unless otherwise noted.

Carbonado diamonds are typically pea-sized or larger porous aggregates of many tiny black crystals. The most characteristic carbonados have been found only in the Central African Republic and in Brazil, in neither place associated with kimberlite, the source of typical gem diamonds. Lead isotope analyses have been interpreted as documenting crystallization of carbonados about 3 billion years ago. The carbonados are found in younger sedimentary rocks.

Mineral grains included within diamonds have been studied extensively for clues to diamond origin. Some typical diamonds contain inclusions of common mantle minerals such as pyrope and forsterite, but such mantle minerals have not been observed in carbonado. In contrast, some carbonados do contain inclusions of minerals characteristic of the Earth’s crust: these inclusions do not necessarily establish formation of the diamonds in the crust, however, because while these obvious crustal inclusions occur in the pores that are common in carbonados, they may have been introduced after carbonado formation. Inclusions of other minerals, rare or nearly absent in the Earth’s crust, are found at least partly incorporated in diamond, not just in pores: among such other minerals are those with compositions of Si, SiC, and Fe‑Ni. No distinctive high-pressure minerals, including the hexagonal carbon polymorph, lonsdaleite, have been found as inclusions in carbonados, although such inclusions might be expected if carbonados formed by meteorite impact.