What causes chatoyancy in non-nacreous pearls?
and their similarities to the chatoyancy caused in other gemstones The “chatoyant effect” of some non-nacreous pearls is caused by the fibrous nature of the nacre-like substance, the needle-like aragonite and calcite microcrystals that form bundles of microcrystalline fibers, whose alignment in a particular way causes an interaction of reflected light that produces the shimmer or the flame structure. This effect is similar to the “chatoyancy” (cat’s-eye effect) produced by bundles of rutile fibers in gemstones like chrysoberyl, beryl, quartz, tourmaline etc. In this case however a luminous streak of reflected light known as the cat’s-eye is produced, which is always perpendicular to the direction in which the fibers are aligned. When the needle like rutile fibers are aligned in two or three directions, in relation to the crystal structure, two or three intersecting bands or streaks of light are produced, and the “cahtoyancy is known as “asterism”, as found in corundum such as star sapp
