Asterism

Asterism is the star: four or six rays of light that glide across a domed stone as it moves under a single lamp. The effect comes from light reflecting off dense, oriented silk, the fine rutile needles laid along the crystal directions of corundum, and it shows only against a curved surface. Every star stone is therefore a cabochon. Star sapphire and star ruby are the classic cases; star garnet, star spinel and star quartz turn up too, for far less money.

Buyers pay for the star before they pay for the stone. Sharpness first. Then centring, because a star sitting off to one side of the dome was cut to save weight rather than to show the effect. Then completeness: six rays running right to the girdle, none broken or faded. Only after that does the body colour beneath the rays come into it, and a translucent red or blue under a crisp star is where the prices climb. A milky grey stone with a fine star is a curiosity. A fine blue with a fine star is a purchase.

On a lab report

Laboratories put the phenomenon in the variety line, "star sapphire" or "star ruby", and word the treatment exactly as they would for any corundum. Read that line closely, because star goods are also produced by diffusion treatment, in which titanium is driven into the surface to grow silk that was never there, and by glass filling of fractured material. Both are stated where found. The treatment line matters here as much as anywhere.

Ask for a current report from a named laboratory, look at the star under one point light rather than a shop's diffuse tubes, and turn the stone slowly. A star that stays sharp, centred and whole from every angle is doing what the price asks of it.