Ruby

 

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 carbuncle, meaning a small coal or ember

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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 color 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 rubies (usually from Burma) often have areas full of inclusions in the form of minute rutile needles (or straws), which interfere 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 strong light, 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.

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