Star garnet (also known as asteriated almandine) is a rare garnet gemstone known for its asterism, or star effect. This usually opaque and deep brownish red or reddish black gem displays its asterism as a four-ray star, or sometimes an even rarer six-ray star. It has only been found in two locations in the world: Idaho (in the U.S.) and India.
The word "garnet" comes from the Latin word "granum" which means "grain." The often rounded garnet shape supposedly reminded its namer of the shape of pomegranate seeds. There are many different varieties of garnet, but star garnet usually comes from the almandine variety. The chemical composition of almandine garnet is iron aluminum silicate. Its crystal structure is isometric forming dodecahedrons, or 12-sided crystals, and can range in size from small sand particles to sizes larger than golf balls.
The asterism is caused by rutile inclusions, or impurities, in the gem. The rutile, a titanium oxide mineral, is called "silk" in the gem world, and is often found positioned along the stone's crystal planes. The amount and arrangement of silk determines if the star will have four or six rays. The star effect is usually weak and requires certain lighting to clearly see it. Bright afternoon sunlight is the best lighting, but a focused light will also work, such as a halogen beam or a small flashlight.
To best show off its asterism, most star garnets are cut in cabochons. (Cabochons are highly polished, rounded, convex forms without facets.) Because of the gemstone's structure, gem cutters have more difficulty orienting its asterism than in other star gemstones, such as sapphire and ruby. Large star garnets are even harder to cut. Most large star garnets have some kind of flaw such as natural fractures, which are sometimes impossible for gem cutters to eliminate.
Believers in the magical and healing properties of garnets think that wearing garnets will help give their owners protection, strength, and healing powers. Some also believe that garnets help with depression and regulate blood flow to the heart.