Idaho's nickname is "The Gem State." It has a long history of producing a variety of gem materials. The most important gems produced to date have been garnet and opal. Notable amounts of jade, topaz, zircon, and tourmaline have also been found in Idaho. Agate, jasper, and petrified wood in many colors and patterns have been found and produced from small deposits in many parts of the state.
Idaho Rhodolite Garnet: This is a photo of a dark purplish red rhodolite garnet from Crystal Creek, Idaho. The stone measures about 9 millimeters across and weighs 2.95 carats. Photo by Bradley Payne of TheGemTrader.com.
Idaho garnet: These almandite-spessartite garnets are from an alluvial deposit in Idaho. They have been transported a short distance from their source rock, and some still retain evidence of their dodecahedral crystal form. They are about four to five millimeters in size and weigh about 0.6 to 0.8 carats each.
Idaho Star Garnet: This is a photo of a four-ray star garnet from northern Idaho. It is a deep purple almandite that is almost black without strong illumination. This stone measures about six millimeters across and about four millimeters high and weighs about 1.5 carats. It is heavily included in addition to the silk that produced the star.
Idaho Garnet
Garnet in Idaho is found in pegmatites, garnetiferous schists, mica schists, and other metamorphic rocks. It can be removed from these rocks or found in the soils that have formed above them. However, most of the garnets that have been produced in Idaho have been from the sands and gravels of placer deposits. This is where they can be most easily produced.
Garnet Abrasives
People have been searching the stream gravels of Idaho for garnets since the late 1800s, but commercial production did not begin until the early 1940s. At that time, thin gravel layers in the sediments along Emerald Creek, Carpenter Creek, and Meadow Creek in the Idaho panhandle were being mined for garnet abrasive. The gravel layers containing the garnet were only a few feet thick, but they consisted of up to 15% almandite garnet by weight. The garnets ranged from sand-sized up to over two inches in diameter. They were being weathered from mica-garnet schist in the upper part of the drainage basins.
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Most of the garnets in these sediments were not gem quality. They were mined and crushed to produce garnet abrasive granules. At some operations, gem-quality stones were picked out by hand before crushing. The deposits were worked from the 1940s through the 1980s and were an important source of industrial garnet in the United States at that time. Today, most abrasive granules used in industry are manufactured rather than natural materials. It is unlikely that garnet mining for abrasive use will return to Idaho in the future.
Gem-Quality Garnet
Gem-quality garnets have been found in the sediments of many Idaho streams. They are mostly deep red almandite and purplish red almandite-spessartite. The important surprise was the discovery of star garnets. These purplish red almandite garnets contain fine rutile needles that produce four-ray stars or six-ray stars when the rough is properly oriented and cut into cabochons. Cutting the garnets to properly produce a star is not easy, and only highly skilled cutters are able to consistently produce them with a nicely centered star.
Star garnets are extremely rare. India and Idaho are the two most famous star garnet localities and the only places where they have been produced in commercial quantities. Small amounts of star garnets have also been found in Russia, Brazil.