Prospector's Guide to Diamonds

Diamonds are a Geologist's Best Friend


During some brief episodes of exploration, >130,000 diamonds were recovered from kimberlites and placers in the State Line kimberlite district along the Colorado-Wyoming State Line. A few diamonds were also recovered from at least one kimberlite in the Iron Mountain district, Wyoming, several diamonds were recovered from a lamprophyre complex at Cedar Mountain, Wyoming, a diamond was recovered from the Homestead Kimberlite in Montana, a diamond was reportedly recovered from the Winkler kimberlite in Kansas, and several placer diamonds were reported in Wyoming, Montana, Colorado, Idaho, California, Oregon and Washington.

A 14.2 carat octahedron recovered at Kelsey Lake, Colorado.

In the State Line district, four different mills were constructed. All were poorly designed such that contamination and security was a problem at all, and all were notorious for rejecting diamonds of all sizes. It is likely that as many diamonds reported to the tailings as was recovered and large diamonds were likely rejected at all of the mills. 

The Kelsey Lake mill in particularly was designed to reject everything larger than 40 carats. But after the recovery of large diamonds, three weighing >15 carats (two >28 carats) and a fragment from a much larger stone, it was realized that the company had made a grave error in cutting costs. The mill was also known to reject diamonds smaller than 40 carats. After the mine had terminated operations, the first sample of tailings tested yielded several diamonds including one >6 carats!

About 50% of the diamonds recovered from the Wyoming kimberlites were high-quality gemstones. About 30% from the Kelsey Lake were gem-quality. Diamonds included transparent white, grey, black, brown, yellow and there were even reports of some with pinkish hue! Based on all of the known kimberlites and all of the known anomalies, this region is a promising target for commercial diamond deposits (especially since it is located adjacent to a US Highway and Interstate system).

Diamond is brittle and has a moderate to high specific gravity of 3.5 with perfect octahedral cleavage. Even though diamond is heavier than water, it is non-wettable (hydrophobic) and will float on water given the right circumstances. Being hydrophobic, diamonds are grease attractive. This property is effectively used to recover many diamonds around the world where extraction tables are coated with grease to extract diamonds. The grease typically is a mixture of Vaseline and paraffin in a 10:1 ratio. 

The specific gravity for diamond is also favorable for concentration in black sands in stream deposits. This, in combination with the extreme hardness of diamond assures survival over great distances. For example, placer diamonds mined along the west coast of Africa in the vicinity of Namibia are assumed to have been carried 500 to 600 miles in the Orange River originating from kimberlites in the Kimberley region in South Africa. The only noticeable damage to these diamonds is the lack of survival of most industrial diamonds. The placers along the west coast are enriched in small, gem-quality diamonds.

While at the Wyoming Geological Survey, our budget was tiny compared to nearly all other government agencies, so we had to rely on a couple of creative people. Jay Roberts was one. Jay created this sophisticated flotation unit by using a plastic orange bucket which was fed water at its base using the hose. The water flowed into the bucket at a slow rate to create a surface tenstion that was like glass like with no ripples. The coffee can was the mineral feed. It contained diamond concentrates and was raised to allow minerals to release at its base and shake off the metal plate into the orange bucket a few millimeters below. Most minerals immediately sank into the orange bucket, but diamond, being non-wettable (hydrophobic) would float off the orange bucket and into the white bucket where they would be captured. This was only used for tiny microdiamond extraction.

Since diamonds are rare, it takes considerable effort and patience to find the gem. It has been estimated diamond occurs in concentrations of less than 1 part per million in commercial diamondiferous kimberlites. This means you have 999,999 parts of waste rock to run through to find 1 part per million of diamond, only after you have searched and searched for the primary host rock. And not all kimberlites, lamproites and lamprophyres have diamond. 

Years ago, it was common knowledge in the mining industry that only about 10% of kimberlites contained diamond (Lampietti and Sutherland, 1978). This estimate is not quite correct as many other rocks at the time had been erroneously classified as kimberlite ? such as lamproite. But since 1978, there have been many additional kimberlites discovered around the world (Hausel, 2008a). So the percentage of diamondiferous verses barren kimberlites is unknown, but likely is slightly higher than that suggested by the 1978 report. At the same time, the percentage of diamondiferous lamproites verses barren lamproites is very low.

The primary host rocks that contain diamond are actually some peridotites and eclogites. These are pieces of the earth?s mantle found as nodules in kimberlite, lamproite and lamprophyre magma. They were picked up by the magma as it rose through the earth?s upper mantle. Many such diamond-rich nodules survive intact after being shot out of the earth?s mantle from depths of 90 to 120 miles, while others dissagregate and their diamond content is diluted in the magma. As an example of how rich some primary host rocks are, one eclogite collected from the Sloan kimberlite in Colorado contained an estimated 20% diamond! The kimberlite magma is many times poorer in diamond than this eclogite.

Figure 6.36. Grease table. Most grease tables vibrate along a vertical axis.

No other mineral (other than gold) seems to evade correct identification by prospectors and rock hounds that diamond: this is mostly due to a poor understanding of mineralogy and other physical properties of diamonds, and because people tend to see things that don?t really exist as they let their imaginations run wild. After 30 years of working with the public and identifying samples for people almost on a daily basis, only two found diamond and correctly identified the mineral. Many hundreds thought they had diamond, but were seeing things they wish were there.