Photos - Gem diamonds from Kelsey Lake, Colorado. Below - vegetation anomaly over kimberlite at Iron Mountain, and blue ground in highwall at Kelsey Lake. DIAMONDS, ASSOCIATED GEMS, & THEIR HOST ROCKS
Diamonds are likely the most valuable commodity on earth based on dollar value per weight. Some diamonds have sold for many times the value of an equivalent weight in gold. A few that stand out for extreme value include some of the rare pink, red & purple diamonds. One, the Hancock Red, weighed only 0.95-carat. It was purplish-red & sold for nearly US$1 million. To put this in perspective, one carat weighs 0.2 gram, which is equivalent to 0.007 ounce. Thus this diamond was valued at >300,000 times an equal weight in gold [1]! Several other pink to red diamonds have also been valued at nearly $1 million per carat. Other priceless diamonds and colored gemstones have been purchased by Royalty or donated to Royal treasuries in that many such diamonds are deemed priceless and are thought to be one of a kind. Most notable of all diamonds were those cut from the Cullinun rough, the largest diamond ever found that weighed 3,106 carats. The extraordinary gems faceted from this huge rough are now proudly displayed in the crown jewels of England.
Diamonds are found with other gems, however few people or companies bother with associated gemstones even though many of these are extraordary and out-shine ruby or emerald. These other gems are known as Cape Ruby (pyrope garnet) and Cape Emerald (chromian diopside and enstatite).
Diamonds have been found in several different rock types, but the primary commercial host rocks are kimberlite and lamproite [2]. Other important rock types are known as lamprophyres.
Kimberlite erupts as a small maar-like volcanoes and dikes. At the surface, these appear as depressions: several have been mistaken for impact craters. However, kimberlite volcanoes typically occur as circular to elongate depressions with vegetation anomalies. They are controlled by fractures, have blue ground (montmorillonite clay) and are so carbonate rich their soils react with dilute hydrochloric acid to give off CO2 gas. Most kimberlites are <0.5 mile in diameter.
Photos - Gem diamonds from Kelsey Lake, Colorado. Below - vegetation anomaly over kimberlite at Iron Mountain, and blue ground in highwall at Kelsey Lake. We found lamprophryes in Montana & Wyoming & identified several hundred cryptovolcanic structures within & 
surrounding the Colorado State Line that are likely diamond deposits (these 
remain unexplored). I ended up mapping the Iron Mountain, State Line, Sheep Rock districts and found several hidden kimberlites in this areas [3,4,5]. Areas that remain unmap include Indian Guide, Twin Mountain, Happy Jack & others. We even found a major district of 50+ kimberlitic anomalies sitting within view of an interstate in the US! Some are the largest kimberlites found in North America. I also mapped the Leucite Hills lamproite field in SW Wyoming, where several lamproites were identified, a few that yielded diamond-stability chromites!
A14.2 ct flawless octedron from Kelsey Lake. Below - carbonate-stained soil over cryptovolcanic structure, and view of one of the Lost Lakes cryptovolcanic structures.
surrounding the Colorado State Line that are likely diamond deposits (these 
remain unexplored). I ended up mapping the Iron Mountain, State Line, Sheep Rock districts and found several hidden kimberlites in this areas [3,4,5]. Areas that remain unmap include Indian Guide, Twin Mountain, Happy Jack & others. We even found a major district of 50+ kimberlitic anomalies sitting within view of an interstate in the US! Some are the largest kimberlites found in North America. I also mapped the Leucite Hills lamproite field in SW Wyoming, where several lamproites were identified, a few that yielded diamond-stability chromites!
A14.2 ct flawless octedron from Kelsey Lake. Below - carbonate-stained soil over cryptovolcanic structure, and view of one of the Lost Lakes cryptovolcanic structures.
Commercial deposits occur in placers, kimberlite and/or lamproite [6]. Several lamprophyres also have diamond. The deposits south of Laramie are kimberlite & placers. Kimberlites were deeply eroded & spilled millions of diamonds into the surrounding streams, but no one has ever systematically looked for diamonds in the creeks!
Kimberlite is a ultrabasic, potassic igneous rock that erupts along fractures from 90 to 120 mi depths. They typically occur in very old cratons (basically ancient continental cores that consist of >1.5 billion year old granite, gneiss & schist). The kimberlite magma rises rapidly from the mantle with considerable water vapor & carbon dioxide in the magma. Some suggest gaseous emplacement velocities are on the order of Mach 3. The eruption is relatively cool: CO2 gas expands to cool the magma such that emplacement temperatures of 32 degrees F are not uncommon. This collection of unusual characteristics results in small, circular maar-like volcanoes (without cones) & dikes that are structurally controlled.
Keep in mind: kimberlite will serpentinize because of water vapor, this produces a soft rock that erodes faster than surrounding country rock & usually results in a depressions with different vegetation than surrounding rocks. These depressions may contain shallow ponds. They are structurally-controlled such that more than one anomaly is often found in a line. Because of calcium carbonate in kimberlite, carbonate will leach into a pond staining the soil white. In the craton basement (i.e., mountain ranges of Colorado, Montana, Wyoming) carbonate sources are rare, so if you spot a structurally-controlled lake surrounded by salt in old Precambrian rock, you might want to find out why.
Kimberlite is a ultrabasic, potassic igneous rock that erupts along fractures from 90 to 120 mi depths. They typically occur in very old cratons (basically ancient continental cores that consist of >1.5 billion year old granite, gneiss & schist). The kimberlite magma rises rapidly from the mantle with considerable water vapor & carbon dioxide in the magma. Some suggest gaseous emplacement velocities are on the order of Mach 3. The eruption is relatively cool: CO2 gas expands to cool the magma such that emplacement temperatures of 32 degrees F are not uncommon. This collection of unusual characteristics results in small, circular maar-like volcanoes (without cones) & dikes that are structurally controlled.
Keep in mind: kimberlite will serpentinize because of water vapor, this produces a soft rock that erodes faster than surrounding country rock & usually results in a depressions with different vegetation than surrounding rocks. These depressions may contain shallow ponds. They are structurally-controlled such that more than one anomaly is often found in a line. Because of calcium carbonate in kimberlite, carbonate will leach into a pond staining the soil white. In the craton basement (i.e., mountain ranges of Colorado, Montana, Wyoming) carbonate sources are rare, so if you spot a structurally-controlled lake surrounded by salt in old Precambrian rock, you might want to find out why.
Diamonds found in Colorado & Wyoming ranged from microdiamonds to 28.3 cts & included one chip from a 80- to 90-ct stone. Some believe there are no commerical deposits in this area, but all 4 mills constructed in the past were so poorly designed they rejected diamonds of any size. For example, the Kelsey Lake mill rejected anything weighing >40 cts! It also rejected most diamonds under 40 cts such that when the tailings were later tested in 1997, the first sample yielded a 6.2-ct stone! The grades of several kimberlites were high, the gem:industrial ratios were good & diamond values were reasonable. The biggest problem with the State Line district was good diamond companies with diamond expertise were in short supply.
One of the >300 kimberlites recently discovered by the author in the US (see lake below). Note the range of colors in the 'kimberlitic indicator minerals' (left).
CAPE RUBIES, CAPE EMERALDS, GARNETS
Associated with diamonds are a host of rare mantle nodules & gemstones known as Cape Ruby (pyrope garnet), Cape Emerald (chromian diopside & enstatite) that are always overlooked by mining companies. Yet these are very attractive, value-added gemstones. With some marketing , these could capture parts of the colored gemstone market. For example, many Cape Emeralds are more saturated & beautiful than emerald. .JPG)
Photos - Faceted pyropes from Green River Basin, Wyoming & Kyanite Eclogite from the Aultman 2 kimberlite, Wyoming.
HOW TO FIND DIAMONDS
Over the years of looking for kimberlite and lamproite, I was amazed at how people continue to over look these rocks and gemstones. On one field trip, I led a group of 50 geologists & prospectors to the Chicken Park kimberlites. They were told we would walk over kimberlites and they were to watch for the kimberlites, as I would not tell them when we walked over the depsoits. We walked over the kimberlites - not one person saw them until I took them back over the rocks to show them what they had missed (and these were considered relatively obvious kimberlites). There are litterally hundreds of kimberlites in the Rocky Mountains of Wyoming and Colorado that remain completely unexplored (I found >400)! So you as a prospector have an opportunity to find a major diamond deposit. Here's how to find the obvious pipes:
Go to a good website such as Google Earth or Virtual Earth which has areal photography [7]. A good place to start would be the Kimberly Region of South Africa. Look for some diamond mines - now look in the areas surrounding the diamond mines (I found several probable kimberlites in this area). Do the same for the NW Territories of Canada (search the Ekati mine). The Ekati has about 120 kimberites in the area surrounding the mine, most are under shallow lakes.
Kimberlites can be recognized by looking for fractures (lineaments). Look at the above image (this is a false color IR image which means the colors are not natural). First, this is located in the craton (all of the rocks under the blue area are Precambrian and greater than 1.5 billion years in age). This is known as the Indian Guide district in Wyoming (west of Chugwater). I found this several years ago. Next note all of the depressions - some form small ponds (water shows up as black in this image). All of these are structurally controlled (note all of the lines (lineaments) that project through the various depressions). Next note that a couple of ponds have white rings (kind of like the bathtub rings we use to leave as kids for our mothers to clean up). On the ground, these will react to weak hydrochloric acid. There is no known source for calcium carbonate in these Precambrian rocks, so these are all likely kimberlites. If you were to visit these on the ground, you would want to look for rounded bounders and cobbles in the depressions (characteristic of kimberlite), and look for blue ground clays, as well as look for the kimberlitic indicator minerals. Also look for diamonds (see page on mineral recognition). So how many kimberlites (or cryptovolcanic depressions) can you find in this one photo? I found at least 25.
Guess what suprises me the most - is that Wyoming has one of the largest kimberlite-lamproite-diamond provinces in the world that extends south into Colorado. Montana has a separate province as does Kansas. So what is wrong with the Colorado and Wyoming governments - here is a potential multi-billion dollar industry that neither is doing a thing to research or promote.
References Cited
