Chrysoberyl

Chrysoberyl is an oxide mineral that crystallizes in the orthorhombic system with a characteristic prismatic habit. Its chemical formula BeAl₂O₄ places it among the hardest compounds after diamond (10 Mohs) and corundum (9 Mohs), making it ideal for luxury jewelry. The crystal structure allows preferential orientation of microscopic rutile inclusions, responsible for the chatoyant (cat's eye) effect that appears as a sharp luminous band when the gemstone is cut en cabochon.

The rarest and most prized variety is alexandrite, named after Tsar Alexander II of Russia. This variety contains traces of chromium (Cr³⁺) that determine its pronounced pleochroism and dichroism: it appears blue-green in daylight or fluorescent light, and red-purple under incandescent light. The most important deposits are found in Brazil (Bahia, Minas Gerais), Sri Lanka, Madagascar, and Russia (Urals). Colorless or yellow-greenish chrysoberyl is more common and less expensive, while gem-quality alexandrite reaches prices comparable to mid-range rubies.

Crystal system: orthorhombic, space group Pmmm (no. 47). Lattice parameters: a = 9.40 Å, b = 5.48 Å, c = 4.43 Å. Mohs hardness: 8.5; density: 3.73–3.85 g/cm³ (varies with chromium content). Refractive index: nα = 1.746–1.748, nβ = 1.748–1.756, nγ = 1.754–1.762 (birefringent, positive optical sign). Birefringence: δ ≈ 0.009–0.010. Dispersion: 0.011 (moderate). Pronounced pleochroism in alexandrite: blue-green/yellow-green/red. UV-Vis spectroscopy: characteristic absorption lines in the red (approximately 680 nm) and blue (approximately 475 nm) for alexandrite, attributed to d³ transitions of Cr³⁺. Fluorescence: reddish under long-wave UV (365 nm) for some specimens. Fracture: conchoidal; no cleavage, but possible twinning along the 110 plane. Thermal stability: stable up to approximately 1800 °C. Solubility: insoluble in dilute acids at room temperature. Common inclusions: rutile (responsible for chatoyancy), mica, feldspar. Natural alexandrite is distinguished from synthetic imitations (often chromium-doped YAG) by Raman spectroscopy and fluorescence analysis.