Star ruby is a rare variety of ruby (Corundum Family) that tends to exhibits asterism (a six-rayed star effect) that shimmers over the surface of the stone when it is moved under a uniform light source. The star effect is due to the aligned needle-like rutile inclusions. It is this rutile needles placed in hexagonal pattern which is responsible for its silky shine.
An asterism or a star effect in a gemstone is an optical phenomenon displayed by some rubies, sapphires, and other gemstones (e.g. garnet, Rose quartz, diopside, spinel) of an enhanced reflective area in the shape of a 4 to 6 ray "star" on the surface of a cabochon cut from the stone. The cabochon cut can be Oval, Round or any other shape.
Gemstones that exhibit special optical phenomena, including asterism (the star effect), chatoyancy (the cat's eye effect), adularescence (shimmering light) and iridescence (the rainbow effect) are categorised as phenomenal gemstones.
Because these phenomenal gems are fairly rare, they naturally attract the attention of rare gems collectors are priced much higher than their normally available counterparts.