Star Garnet

Star garnet is a rare almandine (sometimes almandine-pyrope) garnet that shows asterism — a floating star of light across the dome of a polished cabochon. Most garnets show no star; star garnet's effect comes from light reflecting off tiny needle-like rutile inclusions.

Unusually, star garnet can display a four-rayed star, and occasionally six rays, which is rarer than the typical six-rayed star sapphire.

Gem-quality star garnet is found in only a couple of places worldwide, with Idaho being the most famous — it is the official state gemstone of Idaho.

Star garnet forms as almandine porphyroblasts in mica schists during regional metamorphism. As the garnet crystallizes, oriented needles of rutile (titanium dioxide) grow along its crystallographic directions; when the stone is later cut en cabochon, these aligned needles reflect light into a star.

The garnets weather out of the schist and accumulate in stream gravels, where they are recovered by digging and screening.

The two principal sources of gem star garnet are the Emerald Creek area of Idaho, USA, and parts of India.

Star garnet is recognized by its deep reddish-brown to purplish color and, when polished into a cabochon, a four- or six-rayed star that glides across the surface under a single light.

• Hardness: 7-7.5.
• Crystal system: isometric; singly refractive.
• Streak: white.
• The asterism requires a domed cut and good lighting to see clearly.

Distinguish from star sapphire, which is harder (9) and blue, and from ordinary garnet, which lacks the star. The four-rayed star is a hallmark unique to star garnet.

Star garnet is cut exclusively into cabochons to display its star and is set in rings and pendants prized by collectors. Fine Idaho star garnets are sought-after specimens.

Because gem-quality material is so localized, star garnet has little industrial use and is valued chiefly as a rare collector and jewelry stone.

It holds civic significance as the state gem of Idaho, and rockhounds dig for it at public fee areas near Emerald Creek.