Sapphire 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.
Sapphire is the blue variety of Corundum.
Sapphires can be a very dark blue, to the point of seeming dense and blackish from a distance, sometimes accompanied by a blue to dull green pleochroism, which is only visible from the side in cut stones. They may also be a strong, but not too bright blue, easily recognizable from a distance, this being the real color. Other possibilities are light, usually bright, blue, with the color unevenly distributed; palish blue, or finally, blue with a violet tinge, at least in bright light. Like all corundum, sapphire always has good luster.
Some sapphires display clearly defined streaks of paler color, in contrast to a dark ground. Others have areas with a slightly silky sheen, which are not clearly delineated. Still other, uncommon varieties assume a distinct, milky appearance in strong light, with a marked increase in color intensity.
Inclusions are, as a rule, less obvious in very dark stones, due to their general lack of transparency, whereas medium to large, pale stones often show distinct "veils" or "feathers" caused by very fine inclusions and foreign crystals, which are sometimes transparent, sometimes dark, submetallic, and opaque, and, very occasionally, bright red.
The color and appearance of each sapphire is quite distinctive depending on its source. For example, a deep blue color with distinct blue-green pleochroism and internal streaks straight across or at an angle of 120°, combined with the powerful luster of corundum, indicates a sapphire of Australian origin. A slightly patchy, blue color with imperceptible pleochroism and strong transparency showing veillike inclusions and slight silk effect, still with excellent luster, denotes a sapphire from Sri Lanka. Cornflower to deep blue in a stone without obvious inclusions but of a slightly milky appearance, acquiring a distinct fullness of color in bright light, is characteristic of the rare sapphires from Kashmir.
Of the other blue stones, tanzanite always shows a hint of violet, fairly obvious pleochroism, and less luster than sapphire. Cordierite, apart from being less lustrous and violet gray blue, has striking pleochroism from blue to unmistakable drab yellow. Strongly colored specimens of indicolite tourmaline are often an attractive greenish blue, with a pleochroism ranging from blue to green, but the green is very different from that of sapphire which, when it is present, is always dull or yellowish.
The best sapphires were discovered in a small deposit in Kashmir in 1880, in a remote mountain area which has now been exhausted. Very fine sapphire are also found in Burma, but in limited quantities.
Appreciable quantities of light- and bright-blue sapphire are found in alluvial deposits on the island of Sri Lanka. These are always attractively (if sometimes patchily) colored, the richest versions being very similar to the Burmese sapphires, and equally valuable. The sapphires of Sri Lanka are also famous for the variety on inclusions they display: long, thin, rutile needles, like very fine silk, soft, liquid inclusions arranged in the form of veils, lace, and feathers; striking inclusions with a moving bubble, like a spirit level; zircon crystals with small stress cracks radiating from them, and various other types of transparent crystals.
Sapphires are also mined in Thailand and neighboring Cambodia. These are generally pleasing to the eye, though often rather deeply colored. But most sapphires come from Australia, which has numerous deposits of deeply colored stones, sometimes too dark, in most cases with blue-green pleochroism. These are the least valuable, but most widely available on the market. Less important sources are the USA (Montana), Tanzania, and Malawi.
Sapphire probably derives its name through the Latin "sapphirus" and Greek "sapheiros", from a Sanskrit word. As with other gem names, however, the Latin "sapphirus" did not originally denote the gem it is associated with today. Judging by the description of Pliny the Elder, it almost certainly referred to what is now known as lapis lazuli, rather than corundum.
Sapphires usually are given oval, or less frequently, round mixed cuts, but rectangular or square, step cuts, with or without trimmed corners, are also possible. The cabochon cut is used as well, although less frequently than in the past. Nowadays it is generally reserved for stones full of inclusions or those in which the color is concentrated in a few streaks on a light ground. Stones weighing several carats or even 10 to 20 carats in the case of light-colored specimens, are not uncommon.
The finest stones, weighing at least several carats, are almost as valuable as diamonds and rubies and are hence very highly priced. This is particularly true of most sapphires from Kashmir, many from Burma, and some from Sri Lanka, Cambodia, and Thailand. But when the color is too dark, blackish, or greenish-blue or a bit too pale, the value falls sharply, to that normal for secondary gems. Inclusions obvious to the naked eye also lower the price. Small stones (of a fraction of a carat) are modestly priced and readily available. Large ones (from more than ten to several tens of carats), although not common, are much less rare than rubies of this size.
Sapphire has been imitated by dark to cobalt blue glass, but particularly by doublets with a top part consisting of red almandine garnet, which is very hard and lustrous, with natural inclusions, and a bottom part of dark-to-cobalt blue glass, welded together, not glued. It has also been imitated in the past by synthetic blue spinel, which is brightly colored but emits strange red gleams in bright light.
Synthetic sapphire has likewise been produced for many years now, mainly by the Verneuil flame fusion method. Doublets have been produced consisting of a top portion of light-green or yellow-green natural corundum with visible inclusions and a lower portion of synthetic sapphire, held together by transparent cement. The visible inclusions and typical corundum of the top part, along with the color, make these doublets very convincing at first sight.
Since the end of the 1970s, greater knowledge of the nature and causes of color in gemstones has enabled the modification of this feature by various procedures. One very recent method us to subject very pale blue, almost colorless stones with numerous silklike rutile inclusions to prolonged heating at temperatures in the region of 1500°-1600°C in a reducing environment. This "reactivates" the titanium in the rutile, which reacts with the traces of iron in the sapphire. In this way the silk is absorbed, while the trivalent titanium and iron thus formed, which are responsible for the blue coloration of sapphire, greatly intensify the color of the stone. This treatment is now very widespread and more or less reproduces the sequence of events that occurred when many sapphire crystals were formed. As a result, it is not always possible to distinguish a completely natural sapphire from one whose color has been intensified in this way, and they are treated as one on the market.
It is universally regarded as fraudulent if the treated stones are then offered for sale as natural stones, as is often the case.