Ruby

Ruby is the red gem variety of corundum, aluminum oxide (Al₂O₃), owing its saturated crimson color to trace amounts of chromium substituting for aluminum in the crystal lattice. The red variety of corundum owes its color and strong fluorescence to the presence of Cr, as well as traces of Fe. With a hardness of 9 on the Mohs scale, second only to diamond, ruby forms hexagonal, tabular, or barrel-shaped crystals in marbles, amphibolites, and basalt-hosted deposits worldwide. Primary corundum deposits are subdivided into magmatic and metamorphic types, with metamorphic deposits occurring in marble, mafic and ultramafic rocks, and skarns.

The world's finest ruby comes from Mogok, Myanmar, whose marble-hosted deposits have produced "pigeon's blood" rubies since 600 CE, prized for a saturated red fluorescence unmatched elsewhere. Another significant Burmese deposit is Mong Hsu, currently a major ruby producer, though its color isn't as fine as the Mogok material. Mozambique's Montepuez district has become a leading modern source, while Winza in Tanzania and Longido, where opaque crystals are famously intergrown with green zoisite, remain classic African localities. In Southeast Asia, Chanthaburi (Thailand), Pailin (Cambodia), and Luc Yen (Vietnam) supply basalt- and marble-related rubies, while Jegdalek in Afghanistan and the Hunza Valley in Pakistan yield fine marble-hosted crystals. Madagascar, notably at Andilamena and Vatomandry, and Greenland's Aappaluttoq, part of the ancient Fiskenæsset Complex, round out the world's premier ruby-producing terranes; rubies from the Fiskenæsset Complex are probably the oldest in the world.

In North America, Cowee Valley near Franklin, North Carolina has yielded gem and specimen-grade rubies since the 1890s, some historically worked by Tiffany & Co., typically as small hexagonal tabular crystals. Nicely colored rubies have been mined in the Cowee Valley since 1893, and Tiffany's Co was even involved in the mining operations before commercial mining ended in 1914. Yogo Gulch, Montana, though best known for cornflower-blue sapphire, also belongs to this same corundum family prized by collectors for untreated, well-formed crystals. Other notable sources include Karelia in Russia, the Snezhnoe deposit in the Pamir Mountains, and Mysore in India, source of asteriated star ruby. Collectors seek ruby specimens for their intense red-to-pinkish hue, high refractive index, characteristic silk-like rutile inclusions, and rarity in gem-quality, undamaged crystal form — qualities that make fine mineralogical ruby specimens, distinct from cut gemstones, highly collectible pieces of natural history from some of the world's most storied mining districts.