Gemstones

Intro

A gemstone is any rock or mineral that may be used for ornamentation or jewelry. Gemstones usually are minerals prized for their color, beauty, rarity, and endurance. Typically, they are cut and polished to bring out their natural beauty. Even diamonds must be cut into faceted shapes to really sparkle. Colorado has more than thirty varieties of gemstones including aquamarine, rhodochrosite, amazonite, topaz, and diamonds. The official state gemstone is aquamarine, a beautiful blue mineral mostly found around the 13,000-foot (4,000 m) level on Mount Antero. Gemstone-quality rhodochrosite, the state mineral, is produced from the Sweet Home Mine located in Alma mining district, Park County. The largest faceted diamond sourced in the United States (16.87 carats) was found in Colorado. Other notable gem-quality minerals that have been found in Colorado include garnet, tourmaline, lapis lazuli, turquoise, peridot, sapphire, and zircon.

Agate and jasper, two varieties of cryptocrystalline quartz (chalcedony), are found in many places, as well as prismatic crystals of colorless quartz, smoky quartz, and amethyst (purple quartz). Rose quartz may be found as well, though it does not form crystals with well-defined faces (“euhedral”) unlike smoky quartz or amethyst.

Don’t forget to check out the ‘Media’ tab (right) for some amazing gemstone and mineral photographs!

Details

Colorado hosts several gemstone resources. A few of the more important ones are discussed below. See the resources section of this page for references about these deposits. A great summary of gemstones and other minerals found in Colorado published by the U.S. Geological Survey in cooperation with the CGS (see page 69 for the gemstone discussion) called Development of Industrial Minerals in Colorado.

Aquamarine

The state gemstone is aquamarine which is a blue variety of the mineral beryl found on the upper slopes of Mt. Antero in Chaffee County. Aquamarine is found with several other minerals such as smoky quartz in this area. The minerals occur in pegmatites within the Mount Antero Granite, the youngest of the igneous bodies on Mount Antero. The specimen minerals occur in pegmatites and cavities that mainly cut the Mount Antero Granite itself. Much of the areas that contain aquamarine in outcrop is claimed by private entities and cannot be accessed without permission.

Rhodochrosite

Gem quality rhodochrosite is mined at the Sweet Home Mine in Park County. The Sweet Home Mine, a historic silver prospect, was reopened as a rhodochrosite gem mine in 1991 and subsequently closed and then reopened. The mine has produced several world class specimens of rhodochrosite crystals. The Sweet Home Mine is in Buckskin Gulch and was originally located as a silver mine in 1876. The veins and fractures are hosted by a white Proterozoic biotite granodiorite that is heavily altered around the veins and fractures. The granodiorite was intruded into the surrounding Proterozoic gneiss. The Sweet Home Mine was not successful as a silver mine, however, the large rhodochrosite crystals found in the fracture intersections.

Amazonite

Amazonite is a variety of microcline feldspar that is usually blue or green. Colorado amazonite is special because it may be found as large, well-formed crystals, as opposed to the massive varieties found elsewhere. Much of Colorado’s amazonite can be found in the Pikes Peak granite. The granite provides the lead inclusions that color the microcline blue instead of pink. The Crystal Peak area between Woodland Park and Lake George is famous for its amazonite and other minerals, but it is now mostly private land or is claimed by private entities and cannot be accessed without permission.

Diamonds

Diamonds are formed from pure carbon, one of the most abundant elements on the planet. From ancient times diamonds were sought for their extraordinary hardness) and their brilliance, especially in the colorless transparent gemstone variety. The four main optical characteristics of diamonds are transparency, luster, dispersion of light, and color. In its pure carbon form, diamond is completely clear and transparent. As in all natural substances, perfection is nearly impossible to find. Inclusions of other minerals and elements cause varying degrees of opacity. Gemstone varieties of diamond are usually clear and colorless.

Colorado’s first experience with diamonds was The Great Diamond Hoax of 1872. That was the final year of the USGS Geological Exploration of the Fortieth Parallel directed by Clarence King. One of the leaders of the geological exploration, Samuel F. Emmons, overheard some men talking on the train and became suspicious. He reported back to Clarence King that these men purported to have found diamonds along what is now the Colorado-Wyoming border. Many noted San Franciscans had invested heavily in this supposed discovery. King was concerned that a discovery of diamonds in an area that the survey had studied, would reflect poorly on the geological expedition. He and Emmons visited the site and indeed found rubies and diamonds at the surface, particularly around anthills. Emmons noted that the diamonds and rubies were always in the same proportion and always found in disturbed anthills. King exposed the hoax and achieved national recognition for himself and the expedition.

Diamonds form in nature only under the extreme conditions found in the upper mantle at depths of 150 to 200 kilometers (possibly down to 300 kilometers); pressures of greater than 50 kilobars (50,000 times normal atmospheric pressure) and temperatures of 900 to 1,300 C., and possibly higher. Diamonds are brought to the surface in a peculiar igneous rock called kimberlite. Kimberlites are intrusive bodies that originate in the upper mantle and are injected upward through the upper mantle and the lower and upper crust, eventually reaching the earth’s surface as small volcanic complexes.

In the 1960s, what were considered unusual breccias around the Wyoming-Colorado state line in the Front Range and Laramie Range were recognized as kimberlites. Over the ensuing years, about 100 kimberlites have been discovered in the area from Green Mountain west of Boulder to the north end of the Laramie Range in Wyoming. There are approximately 40 known kimberlite pipes in the State Line District. They range in size from a few feet to nearly 1,800 ft (550 m) in diameter and generally have an ellipsoidal to elongate shape.

Diamonds in the kimberlites of the State Line District were discovered in 1975 by Professor Malcolm McCallum at the Colorado State University. Shortly after the discovery, mining companies began a vigorous exploration campaign in the district. The Kelsey Lake kimberlites were bulk sampled and the grades ranged from 0.5 to 1.0 carat per 100 metric tons; gem quality stones were about 20 percent of the sample — about the same as the commercial diamond mines in South Africa. Open pit mining began in late 1996 on the two largest pipes in the Kelsey Lake kimberlites. The Kelsey Lake Mine was North America’s first commercial diamond producer. In 1998, the mine was put into limited production status and has remained that way since.

Agate

Among Colorado minerals that generate value are varieties of cryptocrystalline quartz. In its various forms, this mineral is referred to as carnelian, chalcedony, onyx, sardonyx, chrysoprase, agate, jasper, petrified wood, among others. It is found in many locations around the state, with petrified or agatized wood occurrences in Arapahoe, Douglas, Elbert, Delta, Moffat, and El Paso counties (to name a few).