Natural Star Gemstones
Star gemstones show asterism: a star of light, usually with four or six rays, that glides across the surface of a cabochon as it moves under a single light. The star comes from fine needle-like inclusions aligned in two or three directions inside the crystal. It appears in several species, most famously in corundum (Al2O3) as star ruby and star sapphire, and also in diopside, garnet and others.
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Gemstone guide
Natural Star Gemstones
About star gemstones
Corundum grows as a six-sided crystal, so its needles align in three directions and produce six-rayed stars; species with other crystal shapes, such as diopside, show four rays. To show a centered star, the cabochon has to be cut with its dome over the crystal's axis. Famous examples include the Star of India, a 563-carat star sapphire from Sri Lanka in the American Museum of Natural History. Some star stones on the market are synthetic or have a diffusion-treated star, so the treatment always matters.
What you'll find in this collection
This collection brings together our star sapphires from Thailand, mostly black with some blue, a star ruby from India and black star diopside, in oval and round cabochons from about half a carat to 4 carats. Most are untreated; diffusion-treated stones are marked on the listing.
How to recognise star gemstones
Use a single point of light, such as the sun or a phone flashlight: the star should be visible and move smoothly as you tilt the stone, and fade under diffuse light. Natural stars are often slightly uneven or wavy, while synthetic and diffusion-treated stars tend to look perfectly sharp and painted on the surface.
Caring for star gemstones
Star ruby and star sapphire are hard and tough; star diopside is softer and should be protected from knocks. Clean all star stones with warm soapy water and a soft brush.
Every stone is natural, and each listing gives its weight, size, origin and treatment, with photos of the actual stone.