Minerals of the American West

The American West is a geological wonderland, shaped by plate tectonics, volcanic activity, and deep hydrothermal systems that have concentrated minerals of every description into an extraordinary diversity of deposits. From the turquoise mines of the Southwest to the gold-bearing gravels of California, the western United States has been central to the story of American mineral collecting.

Arizona is arguably the richest mineral state in the continental United States. Its geological diversity — encompassing ancient Precambrian basement rocks, Paleozoic sediments, and extensive Tertiary igneous activity — has produced a stunning variety of mineral deposits. The copper deposits of southeastern Arizona are world-famous. Bisbee, once one of the largest copper producers in the United States, yielded azurite and malachite specimens of unparalleled beauty. The Lavender Pit and the underground workings of the Copper Queen and related mines produced azurite crystals of deep cobalt blue, azurite-malachite pseudomorphs, and massive malachite that has been carved into decorative objects. Globe and Miami in Gila County were centers of copper production and produced spectacular secondary copper minerals. The Morenci Mine, still active as one of the largest open-pit copper mines in North America, has historically produced excellent malachite and azurite.

Turquoise is perhaps the mineral most closely associated with the American Southwest. Native American cultures of the Ancestral Puebloans mined turquoise at Cerrillos, New Mexico — one of the oldest mines in North America — for thousands of years before European contact. Arizona hosts several important turquoise deposits, including Bisbee turquoise (prized for its deep blue color with dark pyrite matrix), Sleeping Beauty turquoise from Globe (valued for its even, sky-blue color with little or no matrix), and Kingman turquoise. New Mexico's Cerrillos Hills, Chino Mine, and Tyrone district also produce significant turquoise. Nevada has historically been the largest turquoise-producing state, with deposits at Carlin, Cortez, and the Lander County district.

Colorado's mineral wealth is closely tied to its spectacular Rocky Mountain geology. The state has been a prolific source of precious metals since the Colorado Gold Rush of 1859. Central City, Black Hawk, and Georgetown were early mining centers that produced gold, silver, and associated minerals. Leadville, high in the Rocky Mountains at nearly 10,000 feet elevation, became one of the greatest silver camps in American history. The carbonate ore bodies at Leadville produced not only silver and lead but spectacular specimens of cerussite, anglesite, galena, and pyrite.

Colorado is also famous for producing exceptional amazonite — a brilliant blue-green variety of microcline feldspar that occurs in the pegmatites of Pike's Peak. Crystal Peak, north of Florissant, has yielded some of the world's finest amazonite specimens, often associated with smoky quartz crystals.

California's mineral history begins with the Gold Rush of 1848–1855, when the discovery of placer gold at Sutter's Mill triggered a massive westward migration. The Mother Lode region of the Sierra Nevada foothills — stretching from Mariposa County in the south to Nevada County in the north — was the heart of California gold country. The gold occurred in hydrothermal quartz veins crosscutting metamorphic and igneous rocks, and in placer deposits derived from erosion of these veins. Although placer mining was the most accessible, hard-rock mining of the quartz veins produced specimens of gold in quartz matrix that are among the finest in the world.

California also hosts the world's most important deposits of benitoite — the state gem mineral. Found only at the Gem Mine in San Benito County, benitoite (BaTiSi3O9) crystals of gem quality are known from no other locality in the world. These striking sapphire-blue crystals, sometimes called the American sapphire, fluoresce a brilliant blue under shortwave ultraviolet light.

Modern collecting sites in the American West include the various BLM (Bureau of Land Management) open areas where collecting for personal use is generally permitted. The Kofa National Wildlife Refuge in Arizona, the collecting areas around Deming, New Mexico (for thunder eggs and geodes), and the many public land areas in the Basin and Range province offer accessible mineral collecting for enthusiasts.