Diamond

Diamond (C) is the allotropic form of carbon that is thermodynamically stable at high pressures and temperatures, characterized by a cubic crystal structure in which each carbon atom is tetrahedrally bonded to four other carbon atoms, creating an extremely rigid three-dimensional network. This atomic architecture confers upon diamond maximum hardness (10 Mohs) and exceptional chemical stability. From a gemological perspective, diamond is evaluated according to the "4 Cs" (carat weight, color, clarity, cut), with the finest specimens sourced from primary deposits in ultramafic rocks (kimberlites and lamproites) of southern Africa (Botswana, South Africa, Namibia), Russia, and more recently Australia and Canada. Formation requires extreme conditions: pressures exceeding 4.5 GPa and temperatures between 900 and 1300 °C, typically at depths greater than 140 km in the Earth's mantle. Diamonds are brought to the surface by explosive volcanic eruptions that create characteristic kimberlitic diatremes. Beyond natural diamond, since the 1950s synthetic diamonds have been produced via high-pressure high-temperature (HPHT) methods, and more recently laboratory-grown diamonds have been manufactured by chemical vapor deposition (CVD), which have revolutionized the gemological and industrial markets.

Crystal system: cubic (space group Fd-3m, Z=8). Lattice parameter: a = 3.567 Å. Density: 3.52 g/cm³ (natural Type Ia diamond). Hardness: 10 Mohs; Knoop indentation ranges from 7000 to 10000 kg/mm² depending on crystallographic orientation. Refractive index: n ≈ 2.417 (monochromatic, 589 nm); dispersion (ω – ε) ≈ 0.044 (high, responsible for "fire"). Birefringence: absent (isotropic). UV-Vis spectroscopy: characteristic absorption in the near-UV; variable fluorescence (blue, yellow, orange) as a function of defects and impurities (N, B, H). Raman spectroscopy: characteristic peak at 1332 cm⁻¹ (C-C vibration). Thermal conductivity: ~2000 W/m·K (maximum among gemstones). Classification by impurity content (DOND - Diamond Optical Nomenclature Database): Type Ia (dispersed N, colorless-yellow), Type Ib (single substitutional N, intense yellow), Type IIa (N absent, colorless), Type IIb (B present, blue). Characteristic inclusions: olivine, garnet, pyrope, eclogite; "feathers" (internal fractures); "clouds" (micro-inclusion aggregates). Instrumental gemological analysis: UV-Vis spectrophotometry, confocal microscopy, Raman tomography, X-ray fluorescence (XRF) to discriminate CVD and HPHT synthetic diamonds from natural stones. Treatments: irradiation, heating, surface coating.