Ruby

Ruby is the red gem variety of the mineral corundum (aluminum oxide), with its color produced by traces of chromium. The same species in any other color is called sapphire, making ruby and sapphire chemically identical but distinguished by color.

At Mohs hardness 9, ruby is exceptionally durable, surpassed only by diamond and moissanite. Combined with its rarity in fine red tones, this hardness has made ruby a premier jewelry stone for millennia.

The most coveted color is a vivid, slightly purplish red historically called "pigeon blood," exemplified by stones from Myanmar (Burma). Fine untreated rubies of good size rank among the most expensive gemstones on earth.

Ruby forms in metamorphic and some igneous environments where aluminum-rich rocks low in silica are subjected to heat and pressure. Marble-hosted rubies, as in Myanmar's Mogok valley and Vietnam, develop when limestone is metamorphosed and chromium becomes available to color the corundum.

Other deposits occur in amphibolite and basalt-related settings, such as those of Thailand and East Africa (Mozambique, Tanzania, Madagascar). Because corundum is hard and dense, rubies frequently weather out of their host rock and concentrate in alluvial gem gravels, which are the traditional mining source in Sri Lanka and Myanmar.

The chromium that gives ruby its red also causes strain that often limits crystal size, so large clean rubies are far rarer than large sapphires.

Look for a transparent to translucent red stone with very high hardness (9) that scratches glass, quartz, and topaz easily. Ruby commonly shows strong red fluorescence under UV light, and chromium produces a characteristic red glow even in daylight.

Natural rubies usually contain inclusions such as fine "silk" (rutile needles) and crystal inclusions; perfectly clean stones should raise suspicion of synthetics. A hexagonal crystal form and distinct dichroism (purplish-red and orangey-red) help confirm corundum.

Look-alikes include red spinel (softer at 8, singly refractive, no dichroism), red garnet (softer, singly refractive), and red glass or synthetic ruby. Garnet and spinel lack ruby's strong dichroism, and glass lacks its hardness.

Ruby is overwhelmingly a gemstone, set in rings, necklaces and fine jewelry, and it is the traditional July birthstone and a 40th-anniversary gem. Top untreated Burmese stones command record per-carat prices at auction.

Industrially, synthetic corundum (produced since the early 1900s) is used for watch bearings, scientific instruments, and was the medium of the first working laser in 1960. Synthetic ruby remains important in optics and lasers.

Metaphysically, ruby is associated with vitality, passion, courage and the root/heart energy, though these claims are not scientific.