GEMSTONE OCCURRENCES IN COLORADO

The best known gem occurrences in Colorado include aquamarine, the state gemstone (Mt. Antero); topaz and smoky quartz (localities in the Pikes Peak batholith, and Mt. Antero); diamond (State Line kimberlite district); amethyst (Red Feather Lakes, Unaweep Canyon, and elsewhere); turquoise (Hall mine, Villa Grove; King mine, Conejos Co.; Turquoise Chief mine, near Leadville; and mines near Cripple Creek); lapis lazuli (Italian Mountain); and plume agate (Del Norte, Saguache Co.). In addition, peridot occurs in Miocene (?) basalt with peridotite xenoliths south of Herring Park, near the southwest edge of South Park. Faceted peridots up to several carats have been cut, though most of the olivine grains are only a few mm in size. Partly gemmy, blue cordierite occurs in a quartz-feldspar-mica pod in schist above Grape Creek, south of the Arkansas River, Fremont Co. Earlier mistaken for sapphire, the cordierite locality was rediscovered by collectors and gem facetors in the 1980s. Blue corundum (generally too small to be of gem use) does occur near Cameron Mountain and Turret, Chaffee Co.

Specialty gemstones occasionally mined and faceted include rhodochrosite from the Sweet Home mine, Park Co.; barite from transparent crystals found in concretions in Upper Cretaceous shales near Grand Junction and in southeastern Colorado, and from Oligocene clay beds near Stoneham, Weld Co.; gemmy green to brown sphalerite from Creede and from the Camp Bird and Portland mines, Ouray Co; and emerald-green chrome diopside from kimberlites in the State Line district. Small amounts of transparent calcite, fluorite, fluorapatite, phenakite, zircon, and rock crystal (quartz) from various localities also have been or could be cut into gemstones. Small transparent topaz crystals occur with spessartine in 29 Ma topaz rhyolite at Ruby Mountain, Chaffee Co. (believed to be comagmatic with the Mt. Antero Granite). Translucent materials sometimes polished for lapidary use include petrified wood, agate, jasper, and dinosaur bone; amazonite (blue-green microcline); banded “sowbelly” agate with amethyst from Creede; lepidolite and elbaite from the Brown Derby mine, Gunnison Co.; massive pink pyroxmangite (earlier thought to be rhodonite) from the Sunnyside mine near Silverton; and gold in quartz from the Sunnyside mine.