The most valuable variety of corundum is ruby. The name comes from the latin rubrum, “red.” Like the other red stones, it has also been called carbuncle, meaning a small coral or ember.



Appearance:
The color varies from fiery vermillion to violet red, but because rubies are pleochroic, different colors are also found in the same stone; bright or sometimes brick red in one direction, tending to carmine in the other. The brightest red and thus the most valuable rubies (usually from Burma) often have areas full of inclusions in the form of minute rutile needles, which interfere with the light, producing a distinctive silky sheen known as silk. Rubies of this type are not usually more than a few carats in weight. 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 al other stones outstanding for their color. Today, however, this cut is reserved for less transparent stones with numerous inclusions.



Distinctive features:
Rubies can often been distinguished by their immediately visible characteristics: a fairly obvious pleochroism, a distinct brightening of color in strong light, the silk effect, and a considerable luster. While spinal 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 strong 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 totally different shade, but can be very similar, with a pleochroism comparable to that of ruby. It does not, however, brightens in strong light, and this can be sufficient indication to warrant testing 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 that of 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 of the finest qualities come from the Mogok region in Burma. These are most truly vermillion, though they still have a touch of carmine. Thailand however is today the main sources of the ruby. Thai rubies are usually 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 have also 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 other is semi opaque and of very limited value.



Value:
The highest quality, best colored (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 grater weight are extremely rare and fetch a exceptionally high prices. Good quality stones of atleast 2 carats, a bit more violet in color, and usually from Thailand, are still quite valuable. The price falls considerably for stones less than carats 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 rather, different likely color and inferior luster. It has sometimes been imitated by doublets, with top part consisting of garnets, 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. 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 the color which is not perfect, but is normal for the majority of rubies, can be much more convincing than a synthetic ruby.