A bright future for black diamonds

The unusual black form of diamond also known as Carbonado is becoming a more and more popular choice for bespoke jewellery. Naturally formed – though very likely in a different process to its sparklier clear cousin – the black diamond is a polycrystalline gem that is generally found in Brazil and Africa. It is usually black or dark grey, though brown stones are also known. Carbonado tends to be more porous than regular diamonds.

The origins of carbonado

The process by which black diamonds are formed is controversial and still not properly understood. A number of theories are current, and none have gained full acceptance by the scientific community. Carbonados are found in alluvial deposits, and neither Brazilian or African sites are associated with kimberlite – the type of volcanic rock in which diamonds are usually found.

Theories of carbonado origin include: the conversion of organic carbon in the same kind of high-temperature and high-pressure volcanic conditions in which regular diamonds are formed; metamorphosis of carbon under the stress of meteor impact; formation induced by radioactive fission of uranium; and formation in supernovas. None of these has proved entirely convincing, which has only added to the mystery and attraction of black diamonds.

Characteristics of black diamonds

Carbonados are made up of collections or ‘aggregates’ of many smaller black crystals. Chemical analysis suggests that they formed around 3 billion years ago, and yet they are found in younger, sedimentary rocks, again indicating their creation elsewhere. The type of ‘inclusions’ (internal pockets of impurities) common in regular diamonds tend not to be found in carbonados; instead of the pyrope and forsterite found in the earth’s mantle – the layer between the surface crust and molten core – elements typically present in the earth’s crust are detectable.

Black diamonds may not be for everyone, but in the right context they can certainly make a striking and unusual statement.