Rubies

Rubies are a red variety of the mineral corundum, which is aluminum oxide (Al2O3). The red color arises from trace amounts of chromium substituting for aluminum in the crystal lattice. Rubies can range from pinkish red to deep red, with the finest stones often described as pigeon-blood red. Like sapphires, rubies are a form of corundum; the difference is color.

Rubies have a hardness of 9 on the Mohs scale, making them among the hardest natural gemstones and highly resistant to scratches. They crystallize in hexagonal prisms and are typically cut for jewelry either as faceted stones or cabochons. The refractive index is about 1.76–1.77 and their specific gravity is around 4.0. Rubies may show inclusions such as rutile needles, which can create asterisms in star rubies when cut en cabochon.

Occurrence and sources: rubies form in metamorphic and igneous environments and are mined in several countries. Major producers include Myanmar (Burma), Sri Lanka, Madagascar, Thailand, Vietnam, Afghanistan, Australia, and Tanzania. The most celebrated rubies often come from Myanmar’s Mogok region, renowned for vivid red stones.

Treatments and synthetic Rubies: many rubies are heat-treated to improve color and clarity; some may receive other enhancements. Synthetic rubies can be produced by flame fusion (Verneuil), flux methods, or the Czochralski process and have the same chemical composition as natural rubies but are generally less valuable. Applications: rubies are widely used in jewelry and, in the form of chromium-doped aluminum oxide, in laser gain media. Rubies are graded by color, clarity, cut, and carat weight, with color hue, saturation, and tone key to value.