Black diamonds possess a distinct opaque, dark appearance, setting them apart from more common colorless diamonds. They are a unique category within the diamond family, recognized by a particular scientific name.
The Name Carbonado
Natural black diamonds are scientifically known as “Carbonado.” This term originated in Brazil around 1840, coined by Portuguese diamond prospectors. The name derives from the Portuguese word for “burned” or “carbonized,” reflecting their charcoal-like appearance.
Distinctive Properties
Black diamonds exhibit unique physical and chemical characteristics. Unlike most diamonds, natural black diamonds have a polycrystalline structure, composed of numerous microscopic diamond crystals fused together with inclusions of graphite and amorphous carbon. This structure contributes to their opacity, as light is absorbed rather than refracted.
The tightly intergrown structure of these tiny crystals gives carbonados exceptional toughness, often exceeding that of single-crystal diamonds. While sharing the same hardness (10 on the Mohs scale) as other diamonds, their aggregate structure makes them highly durable and resistant to breakage. This toughness historically made them valuable for industrial applications.
Natural Versus Treated
It is important to distinguish between natural black diamonds and diamonds treated to achieve a black color. Natural black diamonds gain their deep hue from mineral inclusions like graphite, pyrite, or hematite, trapped during their formation. These natural stones are opaque and typically have a metallic luster, often with visible internal inclusions. Their scarcity and unique origin contribute to their higher value, ranging from $1,500 to $3,000 per carat.
Treated black diamonds are typically colorless or low-quality diamonds that undergo enhancement processes. Common methods include high-temperature/low-pressure treatments or irradiation, which darken existing fractures and inclusions. Treated diamonds often appear more uniformly black and may allow more light to pass through. While also considered real diamonds, their value is significantly lower, typically ranging from $100 to $300 per carat, making them a more accessible option.
Geological Origin
The geological origin of natural black diamonds is a topic of ongoing scientific discussion, with two primary hypotheses. One theory suggests a terrestrial origin, forming deep within the Earth’s mantle under unique conditions. However, this theory faces challenges because carbonados are not found in typical kimberlite pipes and do not contain the mantle minerals usually found in other diamonds.
A more widely accepted hypothesis proposes an extraterrestrial origin. This theory suggests that carbonados formed in interstellar space, possibly within a supernova, and later fell to Earth as meteorites billions of years ago. Evidence supporting this includes their unique carbon isotope composition and the presence of hydrogen, indicative of formation in hydrogen-rich environments outside our solar system. Carbonados are found almost exclusively in alluvial deposits in Brazil and the Central African Republic, which were once part of the same supercontinent, supporting the idea of a single, ancient extraterrestrial impact event.