Natural crystal of Colorado diamond

Diamonds are formed from pure carbon, one of the most abundant elements on planet Earth. Diamonds, even from ancient times, have been sought for their extraordinary hardness (they are the hardest substance known) 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 King Survey of the 40th parallel. One of the leaders of the geological exploration, S. 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.

Diamondiferous kimberlite near Wyoming/Colorado state line.

Diamondiferous kimberlite near Wyoming/Colorado state line. The white fragment is Silurian limestone.

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 a small volcanic complex.

In the 1960s, the odd “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.

Location map of State Line Kimberlite district and "Diamond Peak"

Location map of State Line Kimberlite district and “Diamond Peak”.

Natural shapes that Colorado diamonds can take. The only cut stone is in lower right. Large crystal on right with brown inclusions is 3.5 carats.

Natural shapes that Colorado diamonds can take. The only cut stone is in lower right. Large crystal on right with brown inclusions is 3.5 carats. Photo credit: Tom Hunn.