Idaho Gemstone Mining
Garnet and opal are Idaho's most important gems, but many others have been produced in the state.
Author: Hobart M. King, Ph.D., GIA Graduate Gemologist
A Diversity of Gems from Idaho
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.
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.
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.
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 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, and North Carolina.
It is highly unlikely that you will ever find a star garnet for sale in a mall jewelry store. Because of their rarity, they are almost never placed in commercial jewelry and most jewelry buyers do not know they exist. Most of the star garnets found go into gem collections, mineral collections, and custom jewelry.
Idaho is the most likely place for you to find a star garnet for sale in a jewelry store. In 1967, the Idaho Legislature designated star garnet as the official state gemstone. This has given the gem a strong local popularity with Idaho citizens and people who visit the state.
Opal has been found at many locations in Idaho, and mines there have produced it since the early 1900s. Most of the production has been from small mining operations and open cuts worked by two or three people. There have also been a number of fee mining locations where individuals can pay a fee, search for opal and keep any that they find.
Today, commercial opal production in Idaho occurs at a few locations near the town of Spencer. The largest producer is Spencer Opal Mines. They mine precious opal and sell rough and cut stones. They also have an area where visitors can search through material trucked in from their mine and keep any opals that they find.
Another mine in the Spencer area is Idaho Opal Mines. They mine opal and sell finished stones and jewelry locally and at the Tucson Gem and Mineral Show. The triplet harlequin and pinfire opals shown on the right side of this page were produced at the Constellation Mine and cut by the owner.
Jasper has been found in many locations in Idaho. Some of them have become popular enough to be widely known. These include Owyhee, Willow Creek, Graveyard Point Plume, and Prudent Man Jaspers.
The most famous jasper from Idaho is Bruneau Jasper, mined in Bruneau Canyon in Owyhee County. Its typical color range is from brown to brownish cream and red to reddish cream, marked with sweeping curves and ovals. The jasper precipitated within the gas cavities, fractures, and breccia voids of several rhyolite flows. It is very colorful and nicely patterned. It is not unusual to see slabs the size of a person's hand sell for $100 or more.
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