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. Carbonados, sometimes called “black diamonds,” are a distinct and unusual polycrystalline material.
By International Gem Society 3 minutes read
black diamond in hexagon setting engagement ring

Color-enhanced black diamond in a hexagon setting, split shank engagement ring. © CustomMade. Used with permission.

Black Diamonds

Black diamonds are considered fancy colored diamonds and are graded differently than the more common white or colorless diamonds. Unlike other fancy colors, such as blues or pinks, black diamonds may cost less than white diamonds, due to a lack of consumer demand. However, tastes do change. In recent years, black diamonds have become increasingly popular, especially appealing to those looking for something very dramatic and different in an engagement ring or other type of jewelry.

black diamond engagement ring in rose gold

Color-enhanced, pear-cut black diamond in a rose gold halo engagement ring with a matching tiara-inspired black diamond band. © CustomMade. Used with permission.

How Do Black Diamonds Get Their Color?

Natural black color in diamonds typically comes from the presence of many small, dark colored inclusions and fractures. Most black diamonds are opaque and have an almost metallic luster, though some may have a translucent “salt-and-pepper” appearance. Some diamonds with a black face-up color may have a dark brown or dark green body color, when viewed from the side.

salt-and-pepper diamond

Black diamond with natural “salt-and-pepper” color. You can see the inclusions within the stone. © CustomMade. Used with permission.

Most black diamonds used in jewelry get their color though various procedures. These include high pressure/high temperature (HPHT) and radiation treatments.

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.

black diamonds - 1.64 ct round

1.64-ct round black diamond, HPHT treated. Photo courtesy of liveauctioneers.com and Jasper52.

The 67.5 carat “Black Orlov” is perhaps the most celebrated natural black diamond.

Carbonados

Carbonado

Carbonado. Photo by SixFingeredMan. Public Domain.

A curious stone often referred to as “black diamond,” carbonado is more accurately described as a polycrystalline or aggregate material of amorphous carbon, graphite, and diamond.

This aggregate material has greater durability than diamond and is used primarily in industry.

Carbonado resembles charcoal. In fact, its name means “burned” in Portuguese, but what burned it may be truly extraordinary.

An Extraterrestrial Material?

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. Instead, they occur in alluvial, sedimentary deposits. 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. However, they do possess traces of nitrogen, hydrogen, and osbornite (a mineral otherwise found only in meteors). This suggests they originated in outer space.

carbonado - black diamonds

Carbonado, approximately 29.7 cts and 22 x 17 mm. Photo courtesy of liveauctioneers.com and I.M. Chait Gallery/Auctioneers.

Carbonado Sources

Carbonados date from 2.6 to 3.8 billion years ago. This might also explain why they only occur in Brazil and the Central African Republic. During that span of the Earth’s history, present-day Brazil and the western coast of Africa may have formed a “supercontinent.” (This pairing actually preceded the better-known Gondwanaland). It’s possible that a diamond meteorite struck the Earth at that time, accounting for carbonado’s current distribution.

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Carbonados from the Central African Republic. These samples date from 3.2 billion years ago. Photo by James St. John. Licensed under CC by 2.0.

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).

Diamonds from a Supernova?

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.

supernova

Supernova remnant W49B, photo by NASA Goddard Space Flight Center. Licensed under CC By 2.0.