Although most black diamonds on the market today are either superheated or irradiated to an almost black color, natural black diamonds do exist, though they are extremely rare. The 67.5 carat “Black Orlov” is perhaps the most celebrated natural black diamond. Whether color treated or natural, however, what defines a diamond is its chemical composition (carbon with minor impurities) and its crystalline atomic structure. That is, the entire diamond can be defined as growing from a cubic or isometric crystal structure.
This aggregate material has a greater hardness than diamond and is used primarily in industry. Carbonado resembles charcoal, and its name means “burned” in Portuguese, but what burned it may be truly extraordinary.
Carbonado might have an extraterrestrial origin, which could account for some of its unusual properties. Unlike diamonds, carbonados are never found in igneous kimberlite rock formed deep within the earth but in alluvial, sedimentary deposits instead. The micro-diamonds present in carbonado (typically smaller than 20 microns or 20 millionths of a meter) lack traces of minerals found deep in the Earth’s mantle, typical of other diamonds, but do possess traces of nitrogen, hydrogen, and osbornite (a mineral otherwise found only in meteors) which suggests they originated in outer space.
Carbonados have been dated from 2.6 to 3.8 billion years old, which also might explain why they have only been found in Brazil and in the Central African Republic. During that span of the Earth’s history, what is now Brazil may have been joined to the western coast of Africa. (This “supercontinent” pairing actually preceded the better-known Gondwanaland). It is possible that a diamond meteorite struck the Earth at that time, accounting for carbonado’s current distribution.
In other types of diamonds with meteoric origins, such as lonsdaleite, the physical impact with the Earth may have played a role in diamond formation and may account for some unusual properties (such as lonsdaleite’s purported hexagonal crystal structure). Researchers Jozsef Garai and Stephen Haggerty have argued that the diamonds in carbonado may have been formed in supernovae explosions, which sent chunks of the material into space and ultimately on a collision course with Earth. The impact of these asteroid-sized diamond bodies with the unique geology and oxygen-poor atmosphere of the Earth 2.6 to 3.8 billion years ago may have formed the polycrystalline material we now call carbonado “black diamonds.”