⭐ Star Stones
Asterism is a star of light that seems to float on the surface of a domed (cabochon) gem. It moves as you tilt the stone or move the light.
Needle inclusions
Microscopic needles, usually rutile (titanium oxide), grow in the crystal in neat, parallel sets. In some stones the needles are hematite, ilmenite or magnetite. Each set reflects light as a bright line at right angles to the needles.
Crossing rays
Sets of needles in different directions make the lines cross. In hexagonal crystals like corundum and quartz, three sets give a 6-rayed star. Two sets give a 4-rayed star, as in diopside, most Idaho garnet and moonstone.
12-ray stars
When two different kinds of inclusions overlap, a stone can show 12 rays. This happens, for example, in some Thai black star sapphires, where rutile and hematite needles sit at offset angles.
Cut matters
The star appears only when the stone is cut as a smooth dome with its base parallel to the needles. Strong single light (sunlight or a penlight) shows it best. Synthetic and diffusion-treated star stones exist, and their stars often look unnaturally sharp and perfect.
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🐈 Cat's-eye stones (chatoyancy)
Asterism has a single-ray relative. When a stone has only one set of parallel needles or fibres, a cabochon shows a single bright band that slides across the dome like light in a cat's eye. The most prized example is chrysoberyl cat's-eye ("cymophane"). Tiger's eye gets its shimmer from fibres replaced by quartz.
