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 their familiar faceted shapes to really sparkle. Colorado has more than thirty varieties of gemstones including diamonds, rhodochrosite, and aquamarine. The largest faceted diamond sourced in the United States (16.87 carats) was found in Colorado. The official state gemstone is aquamarine, a beautiful blue mineral mostly found around the 13,000-foot level on Mount Antero. Other notable gem-quality minerals that have been found in Colorado include amazonite, garnet, topaz, tourmaline, lapis lazuli, quartz crystal, smokey and rose quartz, amethyst, turquoise, peridot, sapphire, and zircon. Agate, chalcedony, and jasper, three varieties of cryptocrystalline quartz, are also found in many places.
Hardness is a primary characteristic of the quality of e a gemstone is hard. Hardness doesn’t necessarily mean that the gems are hard to break, but that they are not easily scratched by other materials in the environment. Rated as a 10 on the Mohs Scale of Hardness — which ranges from 1 to 10—a diamond is the hardest of all naturally-occurring minerals is. This means a diamond may scratch all other minerals and, conversely, those minerals cannot scratch it. Rubies and sapphires—different colored varieties of the mineral corundum — are rated a 9 on the Mohs scale while emeralds and aquamarines—different colored varieties of the mineral beryl—come in at 8 on the scale.
A typical generic dust is composed of minerals with hardness of around 6 and 7. Because of this, any minerals with a hardness of less than 6, even though they can be quite beautiful, are not typically suitable for gemstones because they do not have the endurance for their facets and polish to survive a normal environment. They may easily be scratched by dust and other harder substances.
Alabaster, a fine-grained, compact variety of gypsum used to make elegant vases and other decorative items, is quarried in the foothills northwest of Fort Collins and at a quarry south of Carbondale, Colorado. However, it is not a gemstone because it is so soft (Mohs scale of 2). That is why it is easy to carve with steel tools.
Among Colorado minerals that generate high dollar 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, and El Paso counties (to name a few).
Amazonite [K(AlSi3O8)] 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 crystals, but it is now mostly private land or under claims by local mineral clubs and cannot be accessed without permission.
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 feet 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.