Colorado Gemstone Mining
Most people are surprised to learn that diamonds and many other gems have been mined in Colorado!
Author: Hobart M. King, Ph.D., GIA Graduate Gemologist
A wide variety of gems have been mined in Colorado. For a short time, Colorado had the only commercial diamond mine in North America. The state is also famous for its aquamarine, rhodochrosite, amazonite, smoky quartz, and other minerals. For some of these materials, gem-quality crystals are sought by both gem dealers and mineral specimen dealers - and that pushes up prices.
The most famous locality in the world for superb rhodochrosite crystals is the Sweet Home Mine, located near the community of Alma, Colorado. It was opened in 1873 as a silver mine and was worked on and off for silver until the 1960s.
After the silver mining stopped, activity at the Sweet Home Mine focused on the beautiful red rhodochrosite. Ground-penetrating radar was used to locate cavities in the wall rock of the mine. Some of these cavities contained rhodochrosite crystal clusters that could be sold at retail for thousands of dollars each. Many of the rhombohedral crystal specimens produced at the mine are in museums, schools, and private collections around the world.
Some Sweet Home rhodochrosite is transparent enough to cut wonderful faceted stones (but don't do that to a great crystal specimen!). Translucent material is cut into beautiful cabochons of excellent color. These gems are best used in pendants, brooches, and earrings that will not be subject to impact or abrasion. Rhodochrosite is fragile. It has perfect cleavage in three directions and a Mohs Hardness of only 3.5 to 4.
The Sweet Home Mine is now closed, and material from that locality is very hard to get. There are a few other mines in Colorado that produce rhodochrosite; however, none of them are close to matching the quality of the Sweet Home Mine.
Aquamarine, a gem variety of the mineral beryl, is the official state gemstone of Colorado. Serious prospecting for aquamarine began in the Mt. Antero area in the late 1800s. There, crystals of beryl ranging from water-clear goshenite to deep blue aquamarine can be found, along with a little yellow heliodor and pink morganite.
The best aquamarine finds on Mount Antero have been vugs in granite pegmatite exposed on the eastern side of the mountain at elevations over 12,000 feet. The vugs can contain from a few to a few thousand prismatic crystals. A good vug can contain $100,000 worth of aquamarine. If the crystals are mineral specimen quality or have the clarity and color of top-grade gems, the value can easily go a lot higher.
The challenges of looking for aquamarine on Mt. Antero are its altitude, weather, and remote location. At over 14,000 feet in elevation, the weather can be cold, and the aquamarine hunting season is limited to about three months of summer. Wind, lightning storms, and frequent afternoon rains can be life-threatening at low temperatures and high altitudes.
Mt. Antero has grown in popularity with gem hunters after being featured on television prospecting shows. Today there can be a few dozen claims at any given time on Mt. Antero, and visitors must be careful not to trespass on someone's claim. Many claim holders are very unfriendly to visitors because they have put in a lot of sweat and effort hoping to find a nice cavity of high-value gems.
In addition to Mt. Antero, a few other Colorado locations are known to yield nice aquamarine crystals. These include Mt. White, which is connected to Mt. Antero by a high saddle. Nice specimens are also found on Mt. Baldwin and Mt. Princeton, all peaks of over 12,500 feet and located nearby.
Colorado Amazonite and Smoky Quartz
If you go to a mineral show and see nice green amazonite crystals clustered with a couple crystals of smoky quartz, that specimen was probably collected in Colorado. The amazonite-smoky quartz clusters from Pikes Peak and a few other Colorado locations are very popular and often very valuable specimens. Other Colorado locations include Devil's Head, Pine Creek, Cheyenne, Crystal Park, and Harris Park. The crystals from these different areas are diverse in their shape, color, and associated mineral species.
Native Americans knew about amazonite. They made beads from the material to wear and trade. By the late 1800s, commercial ventures were mining the crystals and transporting them east for preparation and sale. The crystals were and continue to be produced from pockets in granite pegmatites. In the early 1900s, crystal collecting was popularized, and a number of fee mining sites were opened to the public.
The main demand for amazonite and smoky quartz from Colorado localities comes from mineral collectors, but some material still goes to lapidary use. Amazonite is popular cut into cabochons, beads, and tumbled stones. Smoky quartz is sometimes faceted.
The United States has two locations that operated as commercial diamond mines. One is the mine currently operated as a fee-mining site at Crater of Diamonds State Park. The other is the inactive Kelsey Lake Diamond Mine near Fort Collins, Colorado.
Over 100 kimberlites have been found in the Colorado-Wyoming state line area. They range in size from a few feet across to over 1/2 mile. Based upon bulk sampling done in the 1970s and 1980s, many of them contain diamonds, but their grades are only a carat or two per hundred metric tons, with the percentage of gem-quality stones being about 20%. This is too low to support a profitable mining operation.
The Kelsey Lake Mine opened in 1996 and began producing small amounts of diamonds, and it continued to operate until 2002 when the mine was closed because of legal problems. Most of the diamonds produced at the mine were clear, gem-quality stones. About one-third of the stones were one carat or larger in size. When the mine closed, there was an identified resource of 17 million tons of ore with an average grade of 4 carats per hundred metric tons. There is no indication that the mine will restart at any time soon or that another kimberlite will be developed.
|Pictures of Opal|
|Ruby and Sapphire|