Rubíes

Rubíes are red to pinkish-red varieties of the mineral corundum, aluminum oxide (Al2O3). The red color results from trace amounts of chromium substituting for aluminum in the crystal lattice. Pure corundum is colorless; when chromium is present, colors range from pink to deep red. Rubies are among the hardest natural substances, rated 9 on the Mohs scale, and they exhibit a vitreous luster. Some rubies show asterism or chatoyancy when cut as cabochons due to needle-like inclusions of rutile.

Crystallographically, rubies form in the hexagonal system as prismatic crystals. Inclusions and trace elements such as iron and titanium can influence tone and saturation. Star rubies arise from aligned rutile needles that create an apparent six-ray star under direct light. Almost all rubies sold today are treated in some way to enhance color or clarity; untreated stones are comparatively rare and often more valuable.

Natural rubies occur in a variety of geological settings, but they are most commonly found in metamorphic and magmatic environments. Major sources include Myanmar (Mogok), Sri Lanka (Ceylon), Thailand, Madagascar, Mozambique, Afghanistan, Vietnam, Australia, and Tanzania. Rubies can form as primary crystals in rock or be extracted from alluvial deposits in river gravels.

Treatments and synthetic rubies are common in the market. Heat treatment to improve color and clarity is widespread; diffusion treatments and fracture filling with glass have been reported. Lab-grown rubies produced by methods such as Verneuil (flame fusion) or hydrothermal processes are chemically identical to natural rubies but are typically distinguished by disclosure and certification.

Value depends on color quality, clarity, cut, and carat weight, with vivid, evenly distributed reds (often termed pigeon-blood) commanding premium prices. Grading and origin disclosures are provided by gemological laboratories and certification bodies.