Asterism is a star-shaped light pattern (often 4- or 6-rayed) seen on some polished cabochons. It forms when oriented needle-like inclusions reflect light in multiple directions. Ruby and sapphire are well-known examples, but other minerals can show weaker stars. Collectors evaluate asterism by rotating the stone under a single point light source and watching whether the star stays centered.
To check Asterism, use one bright point light (not diffuse room lighting) and rotate the cabochon slowly. Note the number of rays and whether they are crisp or fuzzy, because that reflects how well the inclusions are aligned. Surface scratches can soften or distort rays, so evaluate after cleaning. Record lighting conditions because some stars show best under specific light types and angles.
Asterism vs. chatoyancy Asterism shows multiple rays in a star. Chatoyancy shows a single band of light.
Asterism vs. polishing-line “stars” Polishing lines can create false rays. True asterism stays centered and rotates smoothly with the stone.
Asterism vs. uneven dome A flat or off-center dome can distort rays. A centered dome gives a cleaner star.
Asterism vs. diffuse sheen A star has defined rays. A broad sheen may indicate different inclusion alignment or surface texture.