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.

The specific gravity [?] for Sapphire is 4, its refractive index [?] is 1.76-1.77, and its double refraction [?] is 0.008.