Move a tiger’s eye cabochon under a single light source and a bright line seems to glide smoothly across its surface as you tilt it, following your hand rather than staying fixed in place. Do the same with a star sapphire and, instead of a single line, a six-rayed star appears to float just under the surface. Both effects come from the same underlying cause, just at different scales of organization.
Reflecting off parallel fibers
Chatoyancy, the “cat’s eye” effect, happens when a mineral contains a dense array of parallel fibrous inclusions or fine parallel cleavage channels, and light reflecting off all of them simultaneously concentrates into a single bright band perpendicular to the fiber direction. Tiger’s eye gets its chatoyancy from parallel fibers of crocidolite that were gradually replaced by quartz while keeping the original fibrous texture intact, a process called pseudomorphic replacement closely related to the mechanism behind pseudomorphs generally.
Adding a second set of fibers
Asterism, the star effect, is essentially chatoyancy doubled or tripled: it requires two or three distinct sets of parallel inclusions oriented at specific angles to each other, following the crystal’s own internal symmetry, so that light reflects into two or three overlapping bands that together read as a star. Star sapphires and rubies owe their asterism to tiny needle-like inclusions of rutile, titanium dioxide, arranged along the corundum crystal’s threefold symmetry, which is exactly why the star always shows six rays rather than some other number, the geometry is locked in by corundum’s own crystal structure.
Both effects depend entirely on the stone being cut correctly, as a rounded cabochon rather than faceted, since a flat cabochon surface is what lets the reflected light concentrate into a visible line or star instead of scattering across dozens of flat facets.
