Some rubies show a three-point or six rays asterism or star. These rubies are cut into cabochons to display the effect properly. Asterisms are best visible with a single-light source. And move across the stone as the light moves or the stone turns. These effects occur when light is reflected on the silk. That forms the crystal structure of the stone. The structurally oriented needles inclusions, in a certain way. This is one example where inclusions increase the value of a gemstone. Furthermore, rubies can show color changes though this occurs very rarely. As well as chatoyancy or the cat’s eye effect.
A ruby is a pink to blood-red colored gemstone. A variety of the mineral corundum (aluminium oxide). Other varieties of gem colors corundum are sapphires. Ruby is one of the traditional cardinal gems. Together with amethyst, also sapphire, emerald, and diamond. The word ruby comes from ruber, Latin for red. The color of a ruby is due to the chromium element.
The quality of a ruby depends on its color. Also from cut, and clarity. Which, along with carat weight, affect its value. The brightest and most valuable shade of red called blood-red or pigeon blood, commands a large premium over other rubies of similar quality. After color follows clarity: similar to diamonds. A clear stone will command a premium. A star ruby without any needle-like inclusions is a sign that the stone has probably been treated or synthetic. Ruby is the traditional birthstone for July. And it is usually more pink than garnet. Although some rhodolite garnets have a similar pinkish hue to most rubies.
Star or asterism phenomena
Gem cutters often choose to cut cabochon shapes, when stones are low quality. In such gems and stones when the light falls on the cabochon surface and makes star‐like rays. The phenomena is star or also asterism. There are and 6 rays stars observed normally. This happens when the orientation of the needle like inclusions or silk within the crystal is on more than one axis.
Star rubies untreated from Myanmar (Burma)