A. Diamonds
Of all the gems diamonds seem to hold a special fascination for both children and adults. Perhaps this is due to all the adventuresome stories seen in movies and on television or perhaps it stems from a vague understanding of the tremendous forces that create diamonds and the vexing inability of man with his modern technology to duplicate this feat completely.
Man is not at a total loss in this field. Currently we are producing some 44,000 pounds for industrial use annually by a process developed by H. Tracy Hall for the General Electric Research Labs in the early 1950s. His process involved a mixture of graphite powder and an iron compound placed in a hydraulic press. This press was able to generate a force of more than 1.5 million pounds per square inch! To that was added an electrical current which heated the mix to over 4,800 degrees Fahrenheit. From this was produced low quality diamond grit used widely for industrial purposes as an abrasive.
Gem quality diamonds are another story. Instead of the less expensive carbon sources which are used in the manufacture of industrial diamonds, the feed material for gem quality diamonds is the industrial grit, and the pressure and temperatures must be maintained for long periods of time, up to a week. From this we can obtain gem quality diamonds of up to one carat, but unfortunately the cost of manufacture is higher than the present cost of mining the natural stones.
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Natural diamonds are thought to be formed deep within the earth, probably 90 to 120 miles down within the upper regions of the mantle. Here pressures of 975,000 pounds per square inch and temperatures of at least 2,700 degrees Fahrenheit may cause carbon atoms to crystallize into tetrahedral shapes of great strength. Diamond is more resistant to scratching than any other mineral, only another diamond can mark it. It is resistant to acids and alkalis. It is brilliant and has very high dispersive qualities which result in the flashes of light reflecting from the cut stone.
Dispersion
is the ability of a substance to separate white light into its component colors just as a prism or water droplets forming a rainbow. It also has a relatively high specific gravity, 3.5, which results in it being found in
placer
deposits
, those areas in stream beds where heavy and often valuable particles settle out and collect in quantity.
Diamonds were first found in India and for thousands of years this was the only source. They were not mined but rather found in stream gravel and alluvial deposits. Some famous stones from India with fascinating histories are the Koh-i-nor and the Great Mogul. In the early 1700s diamonds were discovered in Brazil. Men panning for gold found clear pebbles that were later recognized as diamonds! Many of these South American gems were shipped to India to be sold in their markets to Europeans. As India’s sources began to dry up, Brazil became more acceptable in the world’s eyes as a diamond source. And so for a while Brazil was the principal producer of diamonds. Even today many fine gemstones come from there.
The most extraordinary diamond finds have occurred only within the last one hundred years or so. In 1866 a Boer farmer’s son found a shiny pebble on a river bank in South Africa. It turned out to be a 21.5 carat diamond! Adventurers from all over the world descended on the site turning it into a free-for-all not unlike the American gold rush, Diamonds were recovered from the river bank, the surface soil, deeper “yellow ground,” and finally at depths of fifty to sixty feet, the “blue ground.” This rock is the original matrix which the diamonds formed in more than 15 million years ago. Diamonds have also been found beneath the beach sand at the mouth of the Orange River and now mining is being done in the Atlantic Ocean in that area.
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Very few diamonds have been found elsewhere in the world. An occasional gem has been found in the American midwest in glacial deposits or in dunes. And in Arkansas some have been found in their rock matrix similar to the blue ground of South Africa. They have also been found in the Soviet Union, but not much is known about these.
B. Rubies and Sapphires
Two gems which are color variations of the same mineral, corundum, are ruby and sapphire. Rubies contain a small amount of chromium, and sapphires have titanium. They are very hard minerals, 9 on the Mohs scale, and have a high specific gravity, 4.0. This allows them to survive being washed great distances into alluvial placers. An interesting variation of these gems is the “star” form which results when needle-like impurities are aligned at angles of sixty degrees to each other during the formation of the stone. Unlike the highly organized and businesslike enterprises that mine diamonds, the mining of rubies and sapphires is a haphazard, sometimes dangerous endeavor. Most of the rubies come from Burma or Thailand and sapphires come from Sri Lanka, Australia, and surprisingly, Montana in the Unites States. Many of the stones travel rather dubious routes and are handled by smugglers and contraband dealers in open-air markets guarded by local police armed with automatic rifles.
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C. Emeralds
Emeralds are quite different from their sister gems in that they are quite soft, only 7.5 on the Mohs scale. They are the green form of beryl, colored by the presence of chromium or vanadium. If rubies have wild tales to tell, emeralds out-do them every time.10 Like something out of an Indiana Jones story the history of emeralds includes torture, conquistadors, native slaves imprisoned in tunnels, and lost treasure maps. Both of the major emerald producing regions in Colombia have violent and greed-filled pasts and presents. One of the areas had to be closed down due to the astronomical numbers of murders and other crimes there. In addition to the problems created by the human element, the mines are plagued by mining-technique problems. The preferred mode of obtaining the emeralds involves dynamiting the parent rock. Although this does speed up the rock removal process, it also fragments many of the emeralds crystals. As an example, the 632 carat Patricia emerald was uncovered by a dynamite blast which also destroyed what appeared to be an even larger stone! Although emeralds are quite soft as gems go, they are often mined from alluvial deposits. The places they are found, however, are never far from their source or else they would be very eroded. Colombian stones are taken directly from the parent rock. Other countries which produce emeralds are Zimbabwe, Zambia, and Brazil. Their stones are considered, however, to be somewhat inferior in color or size.
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D. The Origins of Mineral Crystals
Although the geologic processes which produce minerals, gems, or otherwise, vary in their specifics, there are some generalities to their origins. Like all rocks they form by either igneous, sedimentary, or metamorphic means.
Igneous
formations would include diamonds, rubies, sapphires, topaz, and garnet. These form when magma wells up through cracks and fissures and is trapped in molten pockets. There, water, dissolved gases, and rare elements combine and crystals begin to grow as the molten mass slowly cools. The rock it forms is a mixture of minerals called pegmatite characterized by its large chunks or crystals. With just the right chemicals present and just the right ambient conditions this pegmatite will hold precious gems.
Sedimentary
formations may contain gems with two separate histories. Some of the sedimentary rocks hold crystals that were formed elsewhere and then were moved by erosional forces to sediment basins where they became part of the deposit. Other gems were formed when groundwater seeped down through the volcanic ash or other sediments dissolving minerals and moving them into pockets. Opal and turquoise are examples of this type of gem.
When tectonic processes, volcanic activity, or deep burial subject rock to great heat and pressure,
metamorphic
forms may result. The partial melting of the parent rock allows specific minerals to escape and move to areas where they concentrate into gems. Examples are garnets, which can be found here in Connecticut, and the famous Burmese rubies and sapphires.
E. Geodes
Geodes are hollow balls of mineral with numerous well-developed crystals lining the walls of the interior. The formation of geodes is not well understood and there are several theories that attempt to explain these unusual shapes. Some geologists feel that they are merely hollow spaces formed by gas bubbles trapped in molten rock when it cooled. This would allow space for the projecting crystals to form but does not explain the ease with which the geode separates from the host rock. Other geologists think the round shape was originally formed as a concretion which later eroded away leaving a hollow space in which crystals grew. Still other researchers put forth a rather complicated geologic scenario involving a hollow of undetermined origin, its filling with fluids, including a gelatinous silica layer which provides a semipermeable membrane for osmotic movement of water into and out of the open space, and as a final step the dehydration of the gel and the subsequent formation of crystals from mineral rich waters that leak into the opening through cracks.
Displaying geodes to the students is an extremely valuable tool as it is obvious to the group that the individual perfectly formed crystals could not have been carved out by man. This idea of the human manufacture of crystals, especially the particularly beautiful quartz or pyrite crystals, is quite common among students at this grade level.