What are known as "black diamonds" may actually be two separate things. Black diamonds used in the jewelry business and black diamonds used in industrial settings for boring have separate origins. Conventional black diamonds (the ones used in jewelry making) are cut from single crystals that will break along a single, natural line. Carbonado black diamonds, however, are formed from millions of small crystals stuck together in a single mass.
Conventional Black Diamond
In the jewelry-making business, black diamonds are generally a poor quality of conventional diamond that is either mined as a black stone (untreated) or is a brown or poor quality diamond that is treated via the process of irradiation to look black. It is believed that an untreated black diamond receives its color from many multiple inclusions of graphite in the stone. Sometimes there are so many graphite inclusions the stone becomes an electrical conductor. The inclusions make polishing the stone difficult. A similar visual result occurs when a poor quality stone is put through an intense irradiation treatment using neutrons. The resulting color is actually a dark bluish green but because the color is so dark, it appears to the eye as black. Viewing a specimen of each type of black diamond will help to inform the novice which is a treated stone and which is not. Holding each up to the light should reveal more transparency in the treated diamond and less in the untreated one.
The Other Black Diamond
Although they were not formed in the same manner, carbonados are also referred to as black diamonds. Unlike conventional diamonds, these black diamonds can only be found in two locations on Earth. Carbonados are more than 3.8 billion years old, whereas conventional diamond formation can be traced to two volcano eruptions that occurred 100 million and 1 billion years ago. Carbonados are extremely difficult to work with in jewelry settings because of how hard they are to cut and polish. These "black diamonds" are so tough that it takes a 20-ton hydraulic press to break them. Failing to scratch one using a white diamond (the hardest stone) should prove the hardness of a carbonado.
Characteristics of Carbonado
Carbonado is only found in central South Africa and Brazil. It has been identified as containing hydrogen, which means that it was created in a hydrogen-based environment like interstellar space. It has been theorized that the carbonado was formed when an asteroid hit the Earth during the same period of time when Brazil and South Africa were still a single continent. Characteristics that can be used in identifying this form of black diamond are color and surface texture. Their color is that of shiny charcoal and their surface is filled with tiny bubbles resembling a combination of melted glass and lava rock. It is believed that the bubbles came from fizzing gas as the carbonados formed, something that is not possible in conventional diamond formation.