Star Sapphire

Star sapphires form in the same geological environments as non-star corundum, primarily in alumina-rich, silica-poor metamorphic rocks. Corundum crystallizes at temperatures of 600 to 800 degrees Celsius under regional metamorphic conditions where aluminum-rich protoliths such as bauxite, laterite, or pelitic sediments are subjected to high temperatures and moderate pressures (4 to 10 kilobars). The critical chemical requirement is an environment depleted in silica, because in silica-rich systems, aluminum combines with silicon to form feldspars and micas instead of corundum.

The asterism that defines star sapphire develops during a secondary process after the corundum crystal has formed. As the sapphire cools slowly from metamorphic temperatures, dissolved titanium that was incorporated into the crystal lattice at high temperature becomes supersaturated. At temperatures below approximately 500 degrees Celsius, the titanium exsolves from the corundum structure and combines with residual oxygen to form microscopic needles of rutile (TiO₂). These needles, called "silk" by gemologists, orient themselves along three crystallographic directions at 60-degree angles to each other, following the trigonal symmetry of the corundum host. Each set of parallel needles reflects light in a band perpendicular to its length, and the three intersecting bands create the six-rayed star.

The quality of the star depends on the density, fineness, and orientation of the rutile silk. The best star sapphires have needles that are numerous enough to create a bright, sharp star but fine enough (typically 0.5 to 2 micrometers in diameter) to allow translucency rather than rendering the stone completely opaque. If the stone is heated to high temperatures during treatment, the rutile silk dissolves back into the corundum structure, destroying the star. This is why star sapphires are never heat-treated for clarity like faceted sapphires. Very rare twelve-rayed stars occur when both rutile and hematite silk are present, each mineral producing its own six-rayed star pattern.

Star sapphires are identified by the six-rayed star pattern (asterism) visible when the cabochon-cut stone is illuminated with a single point light source. The star should appear to float above the surface of the stone and move smoothly as the light source or viewing angle changes. Hardness of 9 on the Mohs scale means star sapphire is scratched only by diamond and synthetic corundum. Specific gravity of 3.95 to 4.10 makes it notably heavier than common look-alike stones.

Distinguish star sapphire from star garnet, which produces a four-rayed star (sometimes six) but has lower hardness (6.5 to 7.5) and different specific gravity (3.5 to 4.3 depending on species). Star quartz produces a six-rayed star but is much softer (hardness 7) and lighter (SG 2.65). The rays in star quartz are also typically broader and less sharply defined. Star diopside shows a four-rayed star and is softer (5.5 to 6.5). Under magnification, examine the silk. In natural star sapphire, the rutile needles should appear as fine, evenly spaced parallel lines in three directions at 120-degree angles. The intersections of the three needle sets should align precisely with the star's center when the stone is oriented correctly.