Asterism
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Definition
An optical phenomenon in which intersecting bands of reflected light form a star pattern — usually four- or six-rayed — across the surface of a cabochon-cut gem.
Detailed Explanation
Asterism requires two or three sets of parallel inclusions or fibers oriented at fixed angles determined by the crystal system. Corundum (ruby and sapphire) grows in the trigonal system and typically shows a six-rayed star from three sets of rutile needles oriented 60° apart. Spinel, being cubic, produces four-rayed stars from two sets of inclusions. For a star to be sharp and well-centered, the stone must be cut with the table precisely perpendicular to the c-axis. Star rubies and star sapphires from Burma and Sri Lanka are among the most prized asterism gems.