Asterism

Asterism is the optical phenomenon in which a star-shaped pattern of intersecting light rays appears on the surface of a gemstone when viewed under a single light source. The star is visible in cabochon-cut (smoothly-domed) sapphire, ruby, and some garnet, diopside, and rose quartz specimens. The phenomenon is caused by reflection of light off oriented inclusions (most commonly rutile silk needles in corundum) that intersect at fixed angles dictated by the host crystal's structure.

Star sapphire and star ruby (corundum-family) show six-ray asterism — three sets of parallel rutile needles oriented along the crystallographic c-axis at 60-degree angles to each other. When the cabochon is properly oriented and viewed under a point light source (sun, halogen spotlight), the light reflects off each set of needles and produces three intersecting rays meeting at the apex of the dome, forming the six-ray star. The star moves as the stone or the light source moves, "floating" across the surface in a way that distinguishes asterism from surface scratches or painted patterns.

The quality of the star depends on three factors: silk density (more silk produces a sharper star), silk orientation (better alignment produces straighter rays), and cabochon orientation (the c-axis of the crystal must be perpendicular to the base of the cabochon for the star to center). Top-grade star sapphires show six sharp, straight, evenly-balanced rays that meet exactly at the dome's apex and remain centered as the stone moves under the light. Lower-grade stars show weak, uneven, or off-center rays.

The Black Star of Queensland (a 733-carat black star sapphire) and the Star of India (a 563-carat blue star sapphire at the American Museum of Natural History) are the historical references. Both show pronounced six-ray asterism under point light. Star sapphire pricing is highly dependent on body color and star quality combined — a sharp star on a body color that ranges from blue to gray to black trades $200 to $2,000 per carat at commercial grades; investment-grade Burmese star sapphire with strong color and sharp star trades $5,000 to $15,000 per carat at 5 ct and above.

Star ruby pricing is higher than star sapphire on average because of ruby's rarity. A clean 3 ct unheated star ruby with sharp six-ray asterism trades $3,000 to $10,000 per carat at commercial grades; top Mogok star rubies have reached $30,000 per carat at auction.

Twelve-ray asterism is rare but documented in some sapphires with two distinct silk generations at offset angles. Twelve-ray stars trade at meaningful premiums to six-ray equivalents.