Ruby is a mineral with a hardness of 9 out of 10 on the Mohs scale of mineral hardness [?]. These Trigonally structured gems are made of aluminum oxide, their full chemical compound being Al2O3.
Ruby is the most valuable variety of Corundum.
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 red 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 coloring pigment is chrome, and, for brown hues, some iron additionally. The red color varies with the individual deposits, so it is not possible to determine the source area from the color, as each deposit yields various tones. The most desirable color is "pigeon's blood," pure red with a hint of blue. The distribution of color is often uneven, in stripes or spots. As a rough stone, ruby appears dull and greasy but, when cut, the luster can approach that of diamond.
Ruby is the hardest mineral after diamond, although only 1/140th as hard, and seven times as hard as topaz, the next on the Moh's scale. However, the hardness varies in different directions.
It has no cleavage, but has certain preferred directions of parting. Because of its brittleness, care must be taken when cutting and setting.
Rubies can often be distinguished from other red gemstones 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.
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. But today, Thailand is the main source of rubies. Thai rubies are usually 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 years.
Ruby derives its name from the Latin "rubeus," "rubrum," meaning "red." Like other red stones, it has also been called "carbunculus," or carbuncle, meaning a small coal or ember. It was not until about 1800 that ruby, as well as sapphire, was recognized as belonging to the corundum group. Before that, red spinel and garnet were also designated as ruby.
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 as "silk." When the silk is not heavy, the stones are clearer, more attractive, and even more valuable. 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, this cut is reserved for less transparent stones with numerous inclusions.
Ths highest quality, best colored and most transparent stones, 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. 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.
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 20th 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.
Because of its great hardness, ruby corundum that is unsuitable for jewelry purposes is powdered and used as a cutting and polishing medium.