The beauty of gemstones comes from their ability to interact with light, resulting in their color, surface qualities, and even optical phenomena. Asterism is one such phenomenon deriving from the ancient Greek word aster or “star.” It appears as a star-shaped concentration of reflected or refracted light from a gemstone and occurs when a suitable gemstone is cut en cabochon, or in a spherical shape.
Asterism can be observed in many different types of gems including corundum, sapphires, rubies, diopside, spinel, garnet, quartz, star orthoclase and beryl. Star gems display bands of light formed from tightly packed rutile or hematite needle inclusions that are oriented at specific angles to one another. One example is star sapphire, whose rutile needles form at 60 degree angles to one another. A carefully cut cabochon of such a stone can reveal a six-rayed star.
The value of a star gem is derived from many attributes, including depth of body color, how defined and centered the star is, and whether all the rays are of the same length. Some star sapphires are world famous, such as the Star of India which can be seen at the American Museum of Natural History in New York. It is a 563 carat grayish-blue gem originally mined in Sri Lanka and is exceptional not only for its size, but also because it displays a second star on its underside.
Star gems are a miracle of nature and, in ancient times, were believed to have magical powers and thought to belong to the gods. Myriad inclusions in a suitable gemstone must align perfectly to produce this spectacular phenomenon.
