Carbonado, often referred to as black diamond, is a form of diamond. Unlike the single-crystal, transparent gems found in jewelry, Carbonado is a polycrystalline aggregate. This structure results in an opaque, charcoal-like appearance that has fascinated scientists since its discovery in Brazil in the 1840s. Its hypothesized origins are intriguing, with theories ranging from formation deep within the Earth to an extraterrestrial genesis involving a supernova explosion or meteor impact. The complex, non-uniform nature of Carbonado necessitates specialized testing methods to confirm its identity.
The Structural Challenges of Identification
The physical characteristics of Carbonado immediately disqualify it from standard gemological examination. Conventional diamonds are single, flawless crystals that allow light to pass through, which is the basis for grading clarity and color. Carbonado is intensely opaque, preventing the passage of light required for visual inspection or simple refractive index measurements.
The material’s polycrystalline structure—a dense fusion of tiny diamond crystals—eliminates the cleavage planes found in traditional diamonds. This makes Carbonado the toughest form of natural diamond, highly resistant to chipping or fracturing. Furthermore, Carbonado exhibits high porosity, with voids making up between 5% and 15% of its volume.
This micro-porous, aggregate structure is why traditional tests fail. Simple diamond testers, which rely on thermal conductivity, may give a positive result because diamond is an excellent heat conductor. However, they cannot differentiate Carbonado from other opaque, black materials or industrial diamond composites. Definitive identification must rely on techniques that analyze the atomic structure and chemical bonding, not just bulk properties.
Specialized Spectroscopic Analysis
Confirming the identity of Carbonado requires advanced laboratory techniques that can peer into its atomic makeup. Raman spectroscopy is used to confirm the presence of diamond. This method uses a laser to excite the material, and the resulting shift in light frequency reveals the vibrational modes of the atoms.
For a pure diamond, the Raman spectrum shows a sharp peak at approximately 1332.5 cm⁻¹, confirming the sp3 carbon bonds of the diamond lattice. Although Carbonado’s tiny crystal size and impurities can broaden this peak, its presence is the definitive signature of diamond. Raman spectroscopy also helps identify other carbon phases, such as graphite or amorphous carbon, which contribute to its black color.
X-ray Diffraction (XRD) analyzes the crystal structure and orientation of the microscopic crystals within the aggregate. XRD involves directing X-rays at the sample; they diffract off the atomic planes, creating a unique pattern that confirms the cubic crystal structure of the diamond. This technique is useful for determining the crystallite size and the overall structure of the aggregate. Together, Raman spectroscopy and X-ray diffraction confirm the fundamental diamond structure and the polycrystalline arrangement.
Quantification of Physical Properties
Beyond structural confirmation, quantifying Carbonado’s bulk physical properties is essential for differentiation. The most telling property is its density, which is measurably lower than that of a single-crystal diamond. While pure diamond has a density of 3.52 grams per cubic centimeter, Carbonado typically ranges between 3.05 and 3.45 g/cm³.
This reduced density is a consequence of high porosity and the inclusion of lower-density minerals within the voids. Specific gravity testing compares the weight of the material in air to its weight in water, quickly assessing this difference. A real Carbonado will fall into the characteristic lower range, distinguishing it from pure diamond and lighter simulants.
Trace element analysis offers further proof of identity, often pointing to Carbonado’s unusual origin. Unlike traditional diamonds, Carbonado contains a peculiar mix of elements, including exotic metals and alloys such as iron, nickel, and titanium nitride. Furthermore, Carbonado exhibits a distinct carbon isotopic signature, showing a light delta 13 C ratio, which is unusual for terrestrial diamonds. These quantified characteristics serve as the final evidence that the specimen is a true Carbonado diamond.