The blue variety of noble spinel was in the past much less widely known and appreciated than the red variety.

Appearance The finest specimens, though rare, have a bright blue color comparable to that of some sapphires and are very attractive, lustrous, and transparent. Almost as pleasing are equally lustrous light violet-blue stones, resembling some sapphires of the same color. Most blue spinels, however, are a deep, dull color tending to sooty gray, albeit with a violet tinge; and their luster and transparency are marred by a touch of cloudiness. These are often cut quite shallow, in an attempt to lighten the color, usually with modest results. Like sapphires, they are normally given a mixed, oval cut.

Distinctive features Fine blue spinels are hard to distinguish at first sight from sapphires of a similar color. The latter may display clear pleochroism, but this is not always the case, particularly with medium to light blue specimens.

Hence lack of pleochroism is not a sure distinction. But there is a marked variation in density, which is easy to establish, and the refractive indices are also different.Cloudy, deep-blue spinels, on the other hand, are quite distinctive.

Occurrence Blue spinel is mainly found in the gem gravels of Sri Lanka and, rarely, in Burma.

Value Attractively colored blue stones, which are rare and not very large, are worth much the same as the red variety, whereas the fairly common, cloudy, less attractive stones are very modestly priced. '

Simulants and synthetics Like synthetic ruby, dark-blue synthetic spinel was one of the first synthetic gems to be manufactured, at the beginning of the twentieth century.

Synthetic stones were not intended to imitate natural spinel, but the much more precious sapphire, after early unsuccessful attempts to synthesize the latter.

Synthetic spinel was colored by adding cobalt (in natural sapphire, the coloring agent is iron). As a result, if one of these stones is exposed to strong, tungsten light, it emits strange gleams of red fluorescence of a type not seen in any other stone, and this can be a valuable aid to recognition. Light-blue synthetic spinel is also produced in consid erable quantities, to simulate another gem: aquamarine.