rubí

Rubí, in gemology, is the red variety of the mineral corundum (aluminum oxide, Al2O3). It ranks 9 on the Mohs scale, making it one of the hardest natural substances. The typical density is about 4.0 g/cm3, and it has a vitreous luster. Rubí crystals form in both metamorphic and igneous environments and can appear as well-formed crystals or as grains in alluvial deposits.

Color is caused primarily by chromium substituting for aluminum; rubí can range from pink to deep crimson, with the most prized stones displaying a vivid red sometimes described as pigeon blood. Inclusions are common; color zoning occurs. Some rubí exhibit asterism (star rubies) or chatoyancy in certain cuts.

Formation and sources: Ruby forms in metamorphic rocks and in igneous environments such as basaltic magmas; alluvial rubies occur when ruby-bearing rocks weather and deposit gem-bearing sands. Major mining areas include Myanmar (Burma), Sri Lanka (Ceylon), Madagascar, Vietnam, and Tanzania; historically important sources include Thailand and Afghanistan.

Treatment: The majority of rubí in trade are heat-treated to enhance color and clarity. Other treatments include diffusion coloring and, rarely, fracture filling with glass; natural, untreated rubí are rarer and typically command higher prices. Distinguishing natural from treated rubí requires gemological testing.

Synthetics: Lab-grown rubí have been produced since the early 20th century using flame fusion and other methods. They are chemically and optically identical to natural rubí, but can be distinguished with specialized equipment. Disclosure of treatment and synthesis is standard practice in trade.

Economy and uses: Rubí is among the most valuable colored gemstones. Value depends on color, clarity, size, cut, and origin; deep, vivid reds with few inclusions are the most prized. Rubí has been used in jewelry for centuries and appears in collections worldwide, with notable stones held by museums and collectors.