The most valuable variety of corundum is ruby. The name comes from the Latin rubrum, “red." Like other red stones, it has also been called carbunculus, or carbuncte, meaning a small coal or ember.

Appearance The color varies from fiery vermilion to violet red, but because rubies are pleochroic, different colors are also found in the same stone; bright or sometimes brick "ed in one direction, tending to carmine in the other. The :olor is also accompanied by marked fluorescence which IS stimulated by ordinary, artificial light and above all, by the ultraviolet rays of direct sunlight. Thus rubies turn brighter red under such ‘light and the purplish ones look "redder." If the color is too pale, they are no longer called rubies, but pink sapphires; if it is more violet than red, they are known as violet sapphires. But it is hard to establish precise limits, as all the intermediate shades are possible. The brightest red and thus the most valuable rubles (usually from Burma) often have areas full of inclusions in the form of minute rutile needles (or straws), which inter fere with the light, producing a distinctive silky sheen known, in fact, as silk. When the silk is not heavy, the stones are clearer, more attractive, and even more valuable. Other, mainly crystalline inclusions are normally found as well. Rubies of this type are not usually more than a few carats in weight. The rare exceptions generally contain copious inclusions. Violet red, sometimes quite dark, rubies come principally from Thailand. The type most often found on the market nowadays, they can be several carats in weight. They are normally clearer, without patches of silk. While good-sized clear stones are found, specimens with many inclusions are commonly sold as well. Rubies are usually given a mixed cut, which is generally oval, but can be round or, more rarely, other shapes. In the past, they were given a cabochon cut, like all stones outstanding for their color. Today, however, this cut is reserved for less transparent stones with numerous inclusions.

Distinctive features Rubies can often be distinguished by their immediately visible characteristics: a fairly obvious pleochroism, a distinct brightening of color in stronglight, the silk, effect (where present), and a considerable luster. While spinel can be a similar color and has a similar luster, it is not pleochroic, turns much less bright in strong light, and never displays the silk effect. Red garnet is not pleochroic and the color does not brighten in strong light; it has a similar luster, but when given a faceted cut often displays dark, blackish areas within the stone. Red tourmaline is usually a completely different shade, but can be very similar, with a pleochroism comparable to that of ruby. It does not, however, brighten in strong light, and this can be sufficient indication to warrant testing its physical properties, which are quite different. The other red gemstones mentioned also differ physically from ruby. Some caution is needed with garnets, which show wide variations in both density, which in some cases coincides with that of corundum, and refractive index, which can coincide with one of the figures for corundum. Garnet; however, is singly refractive, and examples with an index in the region of 1.76 have a lower density than that of ruby.

Occurrence The rubies with the finest color come from the Mogok region in Burma. These are most truly vermilion, though they still have a touch of carmine. Thailand, however, is today the main source of rubies. Thai rubies areusually slightly less attractive, a bit darker with a violet tinge, but they often have fewer inclusions. Rubies are also found in Sri Lanka, but in very small quantities. Often pale,almost pink, they can be attractive, with an appearance that is both brilliant and lively. Small quantities of very fine rubies also come from the area of Cambodia on the border with Thailand, while rather opaque specimens, mainly of inferior quality, are found in India and Pakistan. Tanzania and neighboring countries havealso been mining rubies for a few years. Some of the rubies found in these countries are almost as finely colored as those from Burma, with inclusions similar to rubies from Thailand, while others are semiopaque and of very limited value.

Value The highest quality, best colored and most transin parent stones (usually from Burma), weighing, for example, 3 to 5 carats, can be as valuable as diamonds, or even more so. Very good quality rubies of even greater weight are extremely rare and fetch exceptionally high prices. Good quality stones of at least 2 carats (a bit more violet in color and usually from Thailand) are still quite valuable (particularly the more transparent ones). The price falls considerably for stones of less than a carat, which are too dark in color, and have inclusions clearly visible to the naked eye.

Simulants and synthetics Ruby has very occasionally been imitated by glass, which has a rather different, less lively color and an inferior luster. It has sometimes been imitated by doublets, with the top part consisting of garnet, to provide luster, hardness, and natural-looking inclusions and the bottom part of red glass, fused rather than cemented to the garnet layer. But such imitations are uncommon. Synthetic ruby has been produced from the beginning of the twentieth century and was the first synthetic gemstone to be manufactured on an industrial scale. To make these synthetic stones harder to distinguish‘ from some natural rubies with numerous inclusions, they have sometimes been fractured internally by heating and rapid cooling. More recently, doublets imitating rubies have also been produced in the Orient. The top part of these doublets consists of poorly colored (usually pale green or yellow) natural corundum with obvious, typical inclusions; and the lower part is synthetic ruby, held to the corundum by transparent cement. The effect is highly deceptive: the reassuring presence of natural inclusions and characteristic luster combined with a color which is not perfect, but is normal for the majority of rubies, can be much more convincing than a synthetic ruby.