The famous Black Prince's Ruby in Britain's crown is not a ruby
A ruby is simply red corundum, the same mineral as sapphire, coloured by a trace of chromium. Swap about one aluminium atom in a hundred and the crystal glows red, even fluorescing at 694 nanometres. That glow powered the first ruby lasers. Meanwhile some historic crown rubies turn out to be spinels.
Chemically a ruby is aluminium oxide, a variety of corundum ranging from pinkish to blood red; other gem-grade corundum is called sapphire, and rubies are sometimes dubbed red sapphires. Its name comes from ruber, Latin for red. It ranks among the traditional cardinal gems with diamond, emerald, sapphire and amethyst, and serves as July's birthstone. At 9 on the Mohs scale, it is outdone among natural gems only by moissanite and diamond.
Pure corundum is colourless, because every aluminium ion sits in a very stable arrangement surrounded by six oxygen ions. A chromium ion is larger and its orbitals point differently, distorting that arrangement so the crystal absorbs ultraviolet, violet and yellow-green light. With about one percent chromium the result is red, reinforced by fluorescent emission of red light. In lab-grown crystals chromium can be set ten to twenty times lower, giving what Theodore Maiman called pink ruby. Because the excited crystal lingers briefly before fluorescing, passing 694-nanometre photons through it again and again with mirrors triggers more in-step photons: the principle of the ruby laser.
Value depends on colour, cut, clarity and carat weight. The most prized shade, pigeon blood, earns a large premium. Oddly, a flawless ruby can raise suspicion: every natural stone carries imperfections, including fine rutile needles known as silk, which gemologists use to tell natural stones from synthetics. Nearly all rubies are now treated, usually by heating before cutting, so untreated high-quality stones fetch far more. The costliest ever sold at auction, the Estrela de Fura, went for US$34.8 million.
Historic names mislead. Stones once called Balas rubies, including the Black Prince's Ruby set in the Imperial State Crown, are actually spinels. Some rhodolite garnets can also share a ruby's pinkish tone, though rubies are usually pinker than garnet.
Source: Ruby
