Ruby

Ruby is a gem variety of corundum, an aluminum oxide crystallized in the trigonal system (space group R3̄c) with hardness 9 on the Mohs scale. The intense red color, which can range from pale pink to pigeon's blood red, is caused by isomorphic substitution of Al³⁺ ions with Cr³⁺ ions in the crystal structure: the higher the chromium content, the more saturated the red. Unlike diamond, ruby is pleochroic, displaying chromatic variations when observed from different angles. Geologically, it forms in metamorphic rocks rich in alumina—primarily marbles and gneisses—through recrystallization processes at high temperatures and pressures, often associated with minerals such as spinel, feldspar, and mica. The most renowned deposits are in Myanmar (Burma), where the Mogok mine produces the finest rubies from a gemological standpoint, followed by Thailand, Sri Lanka, Afghanistan, and recently Tanzania. In the contemporary gemological market, an exceptional quality ruby (saturated color, high transparency, absence of inclusions) can exceed the value of diamond per carat.

Crystal system: trigonal, space group R3̄c; lattice parameters a = 4.761 Å, c = 12.991 Å. Hardness: 9 Mohs (relative scale). Density: 3.97–4.05 g/cm³. Refractive index: nω = 1.760–1.768, nε = 1.768–1.776 (birefringence δ ≈ 0.008). Pleochroism: marked, with colors ranging from intense red to red-orange according to crystallographic axes. UV-Vis spectroscopy: characteristic absorption bands at 468 nm and 694 nm (R-line of red fluorescence, laser emission) due to d-d transitions of Cr³⁺. Fluorescence: vivid red under UV light at 365 nm (characteristic of Cr³⁺ in corundum). Typical inclusions: rutile needles (silk), feldspar crystals, mica, spinel. Common treatments: heating in oxidizing environment (up to 1800 °C) to improve color and clarity; surface diffusion with Be to increase color depth. Gemological classification Gübelin/SSEF: distinction between natural, heat-treated, and synthetic rubies (Verneuil, Czochralski). Market value: highly variable based on geographic origin (Myanmar > Sri Lanka > Thailand), chromatic saturation, absence of treatments, weight in carats (non-linear price curves above 5 ct).