This article is about the mineral or gem. Ruby is one of the most popular traditional jewelry gems and is very durable. Other varieties ruby red orange gem-quality corundum are called sapphires. Some gemstones that are popularly or historically called rubies, such as the Black Prince’s Ruby in the British Imperial State Crown, are actually spinels.
These were once known as “Balas rubies”. The quality of a ruby is determined by its color, cut, and clarity, which, along with carat weight, affect its value. The brightest and most valuable shade of red called blood-red or pigeon blood, commands a large premium over other rubies of similar quality. Rubies have a hardness of 9. 0 on the Mohs scale of mineral hardness. Among the natural gems, only moissanite and diamond are harder, with diamond having a Mohs hardness of 10.
Transmittance of ruby in optical and near-IR spectra. Note the two broad violet and yellow-green absorption bands and one narrow absorption band at the wavelength of 694 nm, which is the wavelength of the ruby laser. If one percent of the aluminium ions are replaced by chromium in ruby, the yellow-green absorption results in a red color for the gem. After absorbing short-wavelength light, there is a short interval of time when the crystal lattice of ruby is in an excited state before fluorescence occurs. If 694-nanometer photons pass through the crystal during that time, they can stimulate more fluorescent photons to be emitted in-phase with them, thus strengthening the intensity of that red light. All natural rubies have imperfections in them, including color impurities and inclusions of rutile needles known as “silk”.
