Asterism is the effect of a six-ray or four-ray star on top of a dome of a cabochon gemstones.
In effect, asterism is the same as chatoyancy, except that there are 3 sets of fine parallel fibers lying along the same lateral axes and intersecting at 60 degrees to each other.
In Burmese rubies those fine parallel fibers are the silky inclusions which fluorescence crimson red under UV light.
The most prominent of the 6-Ray Star effect is found in Corundum (Rubies and Sapphires). The beautiful star in Rubies and Sapphires are much sought after by gems and jewelry connoisseurs.
I have cut some star rubies in Myanmar before, being taught by a gemstones cutter in Mogok, the Land of Rubies and Sapphires. We used some oil to rub on the surface of a rough ruby. The stone was rotated until you could find a fuzzy star on one of the rough surface. This should be the apex of the dome of the star. It was easy to find a fuzzy star, but to really cut out an excellent star effect from a corundum you need to have varied and long years of experiences. Rough rubies are very expensive.
Star Diopside, a black gemstone, is relatively unknown to the public, has a unique 4-Ray star which is created by platelets of magnetite instead of parallel intersecting needles.
Rose quartz which crystallizes in the hexagonal system also has a 6-ray Star effect. However, asterism in Rose Quartz is visible in transmitted light rather than reflected light.
Photo credit and sources: palagems.com, Wikipedia.com, stargemstones.com, thaigem.com.