Ruby

Ruby is the red variety of corundum - the same mineral that produces sapphire. The difference is a single element: chromium. When trace amounts of chromium (typically 1-3%) substitute for aluminum in the corundum crystal structure, the result is the most valued red gemstone on Earth.

Corundum forms in aluminum-rich, silica-poor metamorphic and igneous rocks. The critical requirement is that silica must be absent - if aluminum encounters silica, it forms feldspar or mica instead of corundum. This chemical restriction makes gem corundum uncommon, and the additional requirement of chromium makes ruby genuinely rare.

The finest rubies come from the Mogok Valley in Myanmar, where corundum crystallized in marble during regional metamorphism. The marble host rock scatters light internally, giving Mogok rubies a characteristic fluorescent 'glow' that enhances their red color. The term 'pigeon blood' describes the ideal: a pure red with a faint blue undertone and strong fluorescence.

Ruby is identified by its intense red color combined with exceptional hardness (9 on the Mohs scale - only diamond is harder). The red color distinguishes it from all other corundum varieties, which are classified as sapphire.

Distinguish from red spinel (lower specific gravity, singly refractive), red garnet (lower hardness, different crystal system), and red tourmaline (much lower hardness at 7). Ruby shows strong fluorescence under UV light - most rubies glow bright red under long-wave UV, which is a useful diagnostic tool.