This is the variety of topaz most readily available on the market today.

Appearance It has a definite, uniform sky-blue color, usually without any overtones. Often pale, it can be bright or very rarely an intense blue. lt sometimes has a slight gray or even greenish tinge, giving it a lifeless appearance. Gemstones of several carats or even several tens of carat! in weight are relatively common. Furthermore, they are usually wholly or almost free of inclusions. This is often the case where large amounts of material of no great value are available; the less clear pieces are discarded. The molt common cut is the oval, with the crown and pavilion consisting of very many lozenge-shaped facets, but all the mixed cuts, plus the step cut, are used. As with all light-colored gemstones, the value of blue topaz increases with Intensity of color, provided this is attractive and not somber. Like other types of topaz, it cleaves readily and this can affect its durability.

Distinctive features There is, at first sight, some resemblance to aquamarine; but close observation will distinguish the two, as aquamarine always displays a very attractive pleochroism from blue to greenish blue or even bluish green. Topaz is usually a more definite blue, if any-thing with a grayish tone, which certainly -distinguishes from aquamarine. Measurement of the density alone is not enough to distinguish topaz from synthetic blue spinal, which can be very similar in color. Before measuring other physical properties, such as the refractive indices, the stone can be examined under a lens for signs of birefringence. Although faint, a doubling of the facet edges will bi visible in topaz. if this is present, one can immediately rule out the possibility of its being synthetic spinel or glass, both of which are singly refractive.

Occurrence Blue topaz is found in various parts of_Brazil, Mexico and. the United States.. It is mined in Burma (In the Mogok region famous for rubies) and the Soviet Union (chiefly in the Urals and the Kamchatka Peninsula). it le also found in Namibia and Nigeria.

Value Quite low, several times less than that of aquamarine. The ready availability of blue topaz on the market. even in pieces of considerable size, is probably responsible for this.

Simulants and synthetics Because blue topaz is a relatively minor gemstone compared with aquamarine, it is the latter that is imitated by glass or synthetic blue spinel. Although blue topaz has not been manufactured synthetically on a commercial scale, a completely natural-looking blue coloration has been produced during recent years in colorless topaz by means of irradiation. This practice, regarded as legitimate in the trade, unless performed in such a way as to cause appreciable residual radiation (fortunately, very"“uncommon), is becoming increasingly widespread and is one of the reasons for the present abundance of blue topaz.