Alexandrite

Alexandrite owes its fascinating color-changing property to the presence of trace chromium (Cr³⁺) that substitutes aluminum in the orthorhombic crystal structure. Discovered in 1830 in the Russian Urals and named in honor of Tsar Alexander II, it represents one of the most spectacular optical phenomena in the mineral kingdom. The gemstone forms in pegmatitic environments and metamorphic rocks rich in beryllium; the finest varieties come from Russia (Urals), Brazil (Minas Gerais), and Sri Lanka, where the pleochroic effect is particularly pronounced. On the gemological market, high-quality alexandrite reaches prices comparable to rubies, especially when the color change is sharp and transparency is high. Its hardness of 8.5 on the Mohs scale makes it suitable for any type of jewelry, although care should be taken during ultrasonic cleaning.

Crystal system: orthorhombic, space group *Pnma*. Lattice parameters: a = 9.404 Å, b = 5.476 Å, c = 4.426 Å. Density: 3.71–3.73 g/cm³. Refractive index: α = 1.746, β = 1.755, γ = 1.763 (birefringence Δ = 0.017). Hardness: 8.5 Mohs. Cleavage: imperfect along 110. Luster: vitreous. Pleochroism is trichroic: greenish-blue (a-axis), red-violet (b-axis), yellowish-green (c-axis). The absorption spectrum shows characteristic bands at 680 nm and 700 nm due to Cr³⁺. Fluorescence under UV light is variable: reddish under long-wave UV (365 nm), weak under short-wave UV. The color-change phenomenon (*Alexandrite Effect*) is caused by charge transfer between Cr³⁺ and Fe³⁺ under different color temperatures of incident light. Common inclusions include fingerprints, rutile needles, and occasional magnetite crystals. Under polarized light, birefringence is clearly visible. Raman spectroscopy shows characteristic peaks at 378, 502, and 645 cm⁻¹.