Asterism is a star of light that glides across the dome of a gem. Light reflects from fine needles, often rutile, that lie in two or three directions.
In more detail
Star sapphires and star rubies usually show six rays, and occasionally twelve. Some other gems show four. The stone must be cut as a cabochon to show the star. The finest stars are sharp, straight and centered.
On a grading report
Labs name the phenomenon, for example star sapphire.
Related terms
- Silk: Silk is a mass of fine, crossing needle inclusions, usually rutile, in ruby and sapphire.
- Needle: A needle is a thin, elongated crystal inclusion that looks like a tiny rod at 10x.
- Chatoyancy: Chatoyancy is a single band of light that glides across a gem, like the slit eye of a cat. Light reflects from many aligned, needle-like inclusions.
- Sapphire: Sapphire is corundum of any color other than red. The word alone means a blue stone; other colors take a color word, such as yellow sapphire.
- Ruby: Ruby is the red variety of the mineral corundum. Chromium causes its red color.