Blue zircon (also known as starllte) This does not appear to have been known for long, partly because it is often (but not always) obtained by heat treatment of other colored zircons, which is standard practice commercially. The name starlite is not universally accepted.

Appearance The best color (not often seen) is a light, electric blue not found in any other gem, with pleochroism making it look greenish in one direction. It can also be skyblue with less obvious pleochroism, a very soft, pale blue, or a distinctly greenish light blue. it is usually given a zircon-type brilliant cut, which is not always round; also used are rectangular or square, step cuts. Mixed cuts are less frequent. The strong luster is shown to best advantage by the zircon cut and is less obvious in the step cut. Unfortunately, the brittleness of zircon often results in the edqes being slightly damaged and not clear-cut.

Distinctive features The marked birefringence—characteristic of most zircons-—is easily seen with a lens and will distinguish blue zircon from other stones of a simillar color. With a little experience (which is not easy to come by, as this is not a common gem), the color can be seen to be quite distinctive, particularly the electric blue. Blue zircon has the physical properties of high zircon, hence the refractive indices are not easily measured, but it nearly always has a highly characteristic absorption spectrum.

Occurrence Blue zircons come from Cambodia, Vietnam, and Thailand, where they are cut (and, perhape, treated). All stones are therefore sold as if coming from Thaiand. s ,

Value The best quality, electric-blu-e stones, which are relatively uncommon, but not in great demand because not widely known, are worth slightly less than the top secondary gems. Weak blue or blue-green stones are worth e lot less.

Simulants and synthetics The greenish-blue variety has been imitated by synthetic spinel, which is .not, however, birefringent. This gem is not manufactured synthetically.