Topaz

Topaz is an aluminum and fluorine silicate with the formula Al₂SiO₄(F,OH)₂, crystallizing in the orthorhombic crystal system as prismatic crystals that are often well-defined. Its hardness of 8 on the Mohs scale makes it ideal for jewelry, second only to diamond and corundum among natural gemstones. It forms predominantly in granitic pegmatites and acidic igneous rocks, often in association with quartz, feldspar, and muscovite. Colors vary extraordinarily: imperial topaz (yellow-orange) remains the most sought after by collectors, while blue topaz (often treated with irradiation) dominates the contemporary market. The most prized geographic origins include Ouro Preto in Brazil (historically the primary source), Afghanistan, Myanmar, and Pakistan, where specimens of superior gemological quality are found.

Crystal system: orthorhombic, space group Pbnm. Lattice parameters: a = 4.65 Å, b = 8.80 Å, c = 8.39 Å. Density: 3.49–3.57 g/cm³. Mohs hardness: 8 (perfect cleavage along (01)). Refractive index: nα = 1.606–1.629, nβ = 1.609–1.631, nγ = 1.616–1.638 (birefringence 0.010–0.016). Dispersion: 0.014. Marked pleochroism (blue, yellow, pink according to crystallographic axes). Composition varies with the F/OH ratio; fluorine promotes transparency and color stability. Natural yellow color is due to lattice defects and trace amounts of iron (Fe³⁺); blue is induced by high-energy particle irradiation (neutrons or electrons). UV spectroscopy: yellow fluorescence under long-wave UV (366 nm) for untreated specimens. Polarized light microscopy reveals two-phase fluid inclusions typical of pegmatites. Distinguishing natural from synthetic topaz requires FTIR spectroscopic analysis and precise density measurements.