Saturday, October 26, 2013

How to Identify Diamonds in Nature

14.2 carat octahedral diamond from
Kelsey Lake, Colorado (photo
courtesy of Howard Coopersmith).
In addition to the type of diamond we see at weddings, other varieties of extremely hard natural carbon are known including carbonado (polycrystalline) and lonsdaleite (hexagonal). These are all forms of carbon, as is graphite. But carbonado and lonsdaleite are very rare compared to natural diamond (which in itself is very rare). For information on lonsdaleite and other natural forms of hard carbon, refer to Erlich and Hausel (2002). Only crystalline, isometric diamond (the kind of diamonds we buy from jewelry stores) will be considered.

In its simplest form, isometric diamond is an equal-dimensional mineral that may form six-sided crystals known to mineralogists as hexahedrons; but to prospectors, these are simply cubes. A more common habit (form) of diamond is the octahedron. To imagine what an octahedron looks like, visualize an Egyptian pyramid. Now imagine that pyramid surrounded by a crystal clear lake with its refection in the water. It would appear as if two pyramids were attached at the base: this would be an eight-sided octahedron.

Modified octahedron with many more faces.
Many octahedral crystals develop ridges on the octahedral faces resulting in crystals of trisoctahedral or hexoctahedral habit. Partial resorption of octahedral diamonds produce rounded dodecahedrons (12-sided crystals) with rhombic faces. Many dodecahedrons have ridges on the rhombic faces resulting in a 24-sided crystal known as a trishexahedron. Four-sided tetrahedral diamonds are sometimes encountered that are thought to be distorted octahedrons. Another relatively common form of diamond is a macle, or twinned diamond. Diamond macles appear as flattened triangular crystals. It should be obvious that diamonds have many crystal habits, so if you would like to know more about these, it is recommended to read Bauer (1968a) and Bruton (1978).

Most diamond surfaces will have growth trigons (equilateral triangles) and less commonly trigonal pits. And some will have hexagons (6-sided pits or raised areas. You are likely getting the idea that there are many complexities to understanding diamond crystal habits or shapes. But don’t despair. If the above information snowed you, just remember there is a simple instrument known as a diamond detector sometimes called a diamond detective that you can purchase on-line for a minimal price. It will tell you if you have a diamond or not by simply touching the crystal and pressing a button. Nothing could be simpler for a prospector, rock hound and even geologist and gemologist.
Magnified diamond surface showing several trigons.
Diamonds have distinct, brilliant, greasy luster that is likened to oiled glass. Often quartz is mistaken for diamond, but the dull luster of quartz is no match for the brilliant adamantine luster of diamond. Gem-quality diamonds can be translucent to transparent, colorless, green, yellow, brown, black and rarely blue or pink. Opaque and heavily included diamonds (bort) are used for industrial purposes and have little value.

Diamond is brittle, extremely hard (H=10 on the Moh’s scale), with a specific gravity of 3.5, and has perfect octahedral cleavage. Even though diamond is heavier than water, it is non-wettable (hydrophobic) and will float on water given the right circumstances. Some flotation devises have been designed to extract diamond using water’s surface tension. Being hydrophobic, diamonds are also grease attractive. This property is used to recover diamonds in many places around the world, where shaking tables are coated with grease to extract diamonds from concentrates run over grease tables with water. The grease, usually a mixture of Vaseline and paraffin in a 10:1 ratio, is coated on the shaking table surface.

Under ultraviolet light, many diamonds fluoresce pale blue, green yellow, and rarely red. This characteristic of diamond is used in many diamond mills, such as Sortex, which detects fluorescence from diamonds when they are x-rayed.

Since diamonds are extremely rare, it takes considerable effort and patience to find the gemstone. 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 that 1 part per million diamond only after you have searched and searched for the primary host rock. And not all kimberlites, lamproites and lamprophyres have diamond.

Sloan kimberlite, Colorado. This kimberlite has diamonds as well as other
gemstones including Cape Ruby (pyrope garnet) and chromian diopside.
Years ago, it was common knowledge in the mining industry that only about 10% of kimberlites contained diamond (Lampietti and Sutherland, 1978; Hausel, 1998). This estimate was not quite correct as many other rocks at the time had been erroneously classified as kimberlite – and over the years, there has been a lot more kimberlites found (Hausel, 2008a). So the percentage of diamondiferous verses barren kimberlites is much higher than originally thought. But at the same time, the percentage of diamondiferous verses barren lamproites and lamprophyres is very low.

Iron Mountain kimberlite, Wyoming
The primary rocks that diamond is found in are known as peridotite and eclogite. These are rare mantle-derived rocks (nodules) that are actually sampled, or picked up by rare volcanic eruptions (i.e., kimberlite, lamproite and some lamprophyre). Thus the kimberlite, lamproite and lamprophyre magmas accidentally pick up these rare diamond-rich rocks at great depth and bring them to the earth’s surface in rare old volcanoes. Many of the diamond-rich nodules survive intact after being shot out of the earth’s mantle from depths of 90 to 120 miles, while others break up with their diamonds being diluted in the magma. As an example of how rich some primary host rocks are, one sample of eclogite from the Sloan kimberlite in Colorado contained an estimated 20% diamond! But kimberlite magma itself, was many times poorer in diamond than this eclogite.

No other mineral (other than gold) seems to elude correct identification by prospectors and rock hounds. This is because of a poor understanding of mineralogy and because most people tend to see things that don’t exist and let their imaginations run wild. After 30 years of working with the public and identifying samples for people almost on a daily basis, I only had two people who correctly identified diamond. Many hundreds thought they had diamond, but were seeing things that were just not there.

I have many stories about these prospectors and rock hounds that are both educational and interesting. One prospector called me from his truck at Jeffrey City and wanted to know what to do with all of the diamonds he had found. I was impressed: 

"How many did you find?" I asked.

"There are thousands all over the hill side!" He responded.

Being curious, I asked him, “How are you verifying these diamonds?"

"I just scratch the windshield on my truck, and they leave a nice scratch", he responded.

Being a wise ass, I asked, "Are you going to be able to see out your window well enough to drive home?"

He was right, diamonds will scratch windshields! Windshields are made of glass with a hardness of only 5.5 to 6 on the Moh's scale. This means many minerals will scratch windshields including pyrite, feldspar, corundum and of course quartz. One way windshield’s become pitted (scratched) during dust storms is due to all of the fine sand that is blown into the windshield.

A fairly inexpensive design for a grease table constructed by Jay Roberts at
the Wyoming Geological Survey. The light-bluish white coating is the grease
mixture. The problem with this material (particularly on a university campus)
was that we had to buy Vaseline by the case. We often received accusing
expressions from drug store employees assuming we were with a fraternity
planning for some weekend orgy.

Another prospector called and said he had been diamond hunting for years and never found any. After talking awhile he mentioned his method for diamond testing: "I simply put them on an anvil and hit them with a hammer!" He talked about all of the octahedral crystals he had picked up from streams and kimberlites in the Colorado-Wyoming State Line district (where several known diamond deposits occurred) but none were diamonds because they all failed his test!

A giant diamond from Africa - 620 carats in weight. This diamond was actually on display for a short time at Wiseman's
Jewelry in Laramie, where I was able to photograph the extraordinary stone.
I then explained to him about the difference between hardness and mechanical brittleness and that all diamonds will break when struck by a hammer. You could hear that sound of ... well, it sounded like muffled swearing in the background as he hung up.

In another case, I received a phone call from an individual who claimed to have found the largest diamond in the world just west of Cheyenne. According to an unnamed gemologist from Cheyenne, this crystal was pronounced to be diamond. But the gemologist suggested that before the prospector put it on and purchased a large mansion on the French Riviera, he should visit my office in Laramie for a second opinion.

I gave him directions to my office. It’s about an hour’s drive from Cheyenne, so I was surprised when he and his three family members were knocking on my office door about 30 minutes later. They were apparently anxious to cash in their millions.

The diamond discoverer introduced himself as ‘Jack’ and did not give a last name, and without further hesitation, opened a locked brief case chained to his wrist and showed me the ‘Star of Cheyenne’. It was fist size - about the same size as the famous Cullinan diamond. The Cullinan was by far the largest diamond ever found and weighed a whopping 3,006 carats and was recovered at the Premier Mine in South Africa. It was priceless and ended up in the Crown Jewels of England.

Kimberlite? Nope, some prospectors would call this kimberlite, but it is
actually a lamproite.
In the late 1970s, I had met Dr. Arnold Waters. Dr. Waters was at the time, the former Chief Geologist for DeBeers in South Africa and he told me that when the Cullinan was found, it had a distinct cleaved surface where part of the diamond had been broken in two during assent to the earth's surface in a kimberlite magma (volcano). He indicated the other half of the diamond could have been as large or larger, but was never found! Did it break off somewhere at great depth and still many miles deep in the earth? Did it make it to the surface and was missed by the sorters and ended up in the crusher where it made many little diamonds? It’s something to wonder about.

The ‘Star of Cheyenne’ was reluctantly handed to me. As soon as I saw it, I knew what it was, but decided to have a little fun. First I showed them how to test a mineral’s specific gravity by weighing the gem in water and then in air. I determined the crystal to have a specific gravity about 2.7 - too light for diamond (diamond's specific gravity at 3.5 is heavy enough it would show up with garnets and black sands in a gold pan). I also tested the hardness by taking a diamond chip and easily scratching a deep notch in the crystal. This resulted in an immediate protest by the family as they thought I was scratching their priceless diamond.

“Hold on!,” I exclaimed. “If this were a diamond, I wouldn’t be able to scratch it with a diamond chip, diamond has a Moh’s hardness of 10 and is the hardest known natural mineral, and it is very, very difficult to scratch a diamond with another diamond”. After I calmed them down and convinced them that they had an ordinary piece of rock crystal (transparent massive quartz), they left the office dejected and drove back to Cheyenne with visions of mansions and Porches fading. And I thought this was over.

The next day, I was contacted by one of our other geologists - Ray Harris (RIP) - who stopped in my office to tell me he had just received a call from a person in Cheyenne who had a probable diamond that he wanted to have verified. The person on the phone explained to him that they had already talked to me, but he and his gemologist decided that another opinion was necessary.

The diamond detective - yep, that's me when I was VP of US Exploration for DiamonEx Ltd, Australia. During exploration
 in Colorado, Montana, Kansas and Wyoming, we identified hundreds of cryptovolcanic structures that look almost like
 impact craters with the exception that these are structurally controlled - located on a fault or similar feature. Would you
 like more information from the Gem Hunter? Follow me on Facebook and link to my GemHunter website.
Ray went back to his office to await the family. I laughed to myself. Ray was a very good geologist, but he had a reputation as a klutz. He was famous for running into things, breaking things, and if anything could go wrong – leave it to Ray. One of my favorite stories about Ray took place at a staff meeting. Ray was holding a cup of coffee in his left hand. I notice this and decided to catch him off guard. So I quickly asked him for the time of day. Without hesitation, Ray rotated his wrist to look at his watch pouring his coffee into his lap. We all had a great time with Ray, but during his last year, he was bullied by his supervisor until he died. I will always miss Ray – he was a good friend.
Prismatic quartz crystal from
Hot Springs Arkansas
Anyway, the Cheyenne family arrived with their gem. They talked about the gemologist’s opinions and their concern about my scratching the diamond. I don't know if Ray had ever seen a diamond in the rough before (few geologists had) and after examining the fist-size specimen with his hand lens, he decided to get a better look at the gemstone and carried it to his microscope in his adjacent lab with the family following him. Then it happened! He lost control of the sample and it crashed onto the floor shattering into dozens of pieces.

Blue quartz from Montana
Ray told me the family turned white as ghosts. But Ray consoled them looking down at all of the pieces. “Well, guess it wasn’t diamond – it has conchoidal fracture”. The family scooped up the fragments of their precious quartz and went home, never to be seen again. When Ray told me about his event – I laughed, and pointed out to him that diamond (as well as quartz) has conchoidal fracture. Ray turned white. But don't worry - it was just a piece of quartz.

Sweetwater agates (quartz) from the
Granite Mountains, Wyoming