<Diamonds are infinitely rarer than gold or silver.
They are, technically, a mineral-a naturally occurring solid with a more or less predictable chemical formulas and crystalline structure. Quartz and mica are minerals; most rocks, agglomerations of minerals. Diamond, like the mineral graphite, is made of the single element carbon but is rearranged by heat and pressure into a lattice that is totally different-supremely lustrous,hard, and heavy....
The ones prospectors want-big, clear, and concentrated in minable quantities-are the ones in our story. We should not pretend ewe know exactly how they form; ideas change from time to time. But lab scientists have analyzed minerals found alongside diamonds, broken down separate minerals encased inside (flaws or "inclusions", and synthesized diamonds themselves. As a result, we believe most gem-quality stones come from below the nuclei of continents, in earth's most ancient, stable regions, called cratons. It is said they form 75-125 miles down, in a "diamond stability zone" of 1,700-2,300 degrees Fahrenheit, at 45,000-60,000 atmospheres, in two rock types, eclogite and peridotite. The carbon source is probably primordial fluids, gases, or semisolids-nothing as pedestrian as the proverbial hunk of coal. Crystallization may take a second or 10 million years; we do not know. Crystals may be stored for eons; some may have dated at 3.3 billion years, more than three-quarters of the earth's history.
Diamonds form mainly under cratons because under much of the rest of earth the geothermal gradient is too high; that is, as depth increases, temperature goes up too steeply. Most of the near-surface has been ravaged repeatedly by diamond-unfriendly melting events-rifts, plumes, volcanoes, continental collisions. As a result the lithosphere-the stiff, congealed stuff of tectonic plates floating over earth's hot, gooey asthenosphere-averages only sixty miles deep. However, within the unmoving cratons, things have long been insulated from upset; here cooled, solidified lithospheric roots may penetrate the hot interior as much as 250 miles,looking in cross section like the teeth in your gums. Toward the bottoms, diamonds form. The very oldest cratons are called archons, for the Archean eon, which ended 2.5 billions years ago. They underlie parts of Africa, Siberia, India, Brazil, Greenland, Scandinavia, Canada, and the United States-all places where diamonds may be found.
For the most of history, no one knew where diamonds came from; they were found scattered in a few riverbeds. Then, around 1870, miners in South Africa discovered the means by which they emerge: small deep-seated structures called kimberlites. These are semimolten, gassy eruptions, often starting within or below the diamond stability filed and tearing up through mazes of faults, fissures, and weaknesses. They come on worldwide waves. No one knows why, nor has anyone seen one erupt; the last of seven known waves ended 50 million years ago. We surmise one is like a tornado, traveling upward at twenty or thirty miles an hour, smashing through through innumerable layer, ripping things out on the way and creating a supercharged cereal of liquids and solids. If by chance it intersects diamonds, they may come along; but considering the vast assemblage of things a kimberlite may pick up, diamonds form a minor part, if any at all. And, if the journey takes too long, gets too hot, or too much oxygen gets in, the gems will burn; after all, they are carbon, subject to the same laws as charcoal briquets. This makes diamonds even rarer.
Most kimberlites probably never hit the surface; they sit far below like blind, unblinking eyes. But if one gets near enough, it is progressively liberated from the pressure of overlying rock. At top, it expands and speeds, breaks through, and rockets into the sky to blow out a deep, carrot-shaped crater. Lava rains back in like a souffle that has exploded and settles into itself to solidify into a kimberlite pipe. Each one is usually just a few acres on top, but often they emerge from their complex, tapered plumbing in clusters from six to forty. Eons later, humans dreaming of clear, tiny stones go looking for them.
All the diamonds found in rivers during most of human history probably would not fill more than a wheelbarrow. But in the ten years after pipes were discovered, diamond production multiplied ten times over. Since 1880 it has mutliplied forty times again, and the pace of discovery and mining has increased ever faster. Humanity has now mined over 500 tons-a third of them in the 1990s.
In some ways, diamond hunting is like any other prospecting. Pick an area that, for whatever reason, appears prospective. Then dig gravel, soil, or rock in many spots and search it for footprints of an orebody. Ores of zinc, lead, or copper may erode from a mass and leave a trail of intact rocks ("float"), an invisible chemical trail, or pure particles of the substance itself, which may be followed. Gold is, in fact, where you find it. Like diamond and most other valuable minerals, gold is heavier and more durable than your average substance. Fragments of such heavy minerals may survive long journeys in streambeds or glaciers. Swirl a panful of sand with some water, and light, usually worthless, grains like quarts float off; in the middle, gold flakes, and other heavies settle.
Once you find footprints,it is time to track them to the source. Following up rivers is standard, for they conveniently concentrate debris from identifiable watersheds. Somewhere upstream the material may suddenly peter out, which means you have just passed the first-order tributary feeding it in. Go back, pick up that tributary. And so on. The closer you get, the more and bigger the float, mineral grains, or other signs. At the spot itself, you may see metals inside bedrock leaching out to oxidize in the air, creating a bright rusty stain, or gossan. Chunks of pure, so-called native, copper may lie about. Visible gold may sparkle in a quartz vein. Dig. You have hit the mother lode; you and your descendants will live in splendor of generations.
That is the way it is supposed to work, anyway; it rarely does. Mountain ranges rise and fall, and rivers change their courses. Oceans, lakes, volcanoes, and ice sheets may smear the trail or bury it altogether. You have to be smart and well informed to make the right adjustments-and in the end, lucky. Keep an eye out for followers, bandits, backstabbers,and claim-jumpers; the scientific complications of prospecting are rarely shown in old movies like "Treasure of the Sierra Madre", but the treachery and madness are real.
Diamonds prospecting is the worst. Not only are diamonds rarer than gold and kimberlite more obscured than gold ore, but most kimberlite contains no diamonds; and when it does, they are vanishingly rare. Gold ore grades are measured in ounces per ton, up to a pound per ton. A diamond carat, based on the weight of a tiny dried locust tree seed, equals one-fifth of a gram, and rich deposits are measured in carats per 100 tons- that is parts per million. Even Russia's fabulously rich Mir Pipe had only about 60 carats per 100 tons-three quarters heavily flawed industrial stones. (Gem-quality diamonds are so much more valuable that they typically earn 90 percent of a mine's income.) Given this, only about 30 of the world's 6,000-some known kimberlites have ever become major mines. Even when companies think they have a mine, they may test-dig for months or years to know for sure.
Clearly, you cannot find diamond deposits by looking for diamonds; they are too rare. Instead, you must look for surrogates, secret signs. The most useful are other rare heavy minerals inhabiting kimberlite in quantities 100,00 times greater -variously called diamond indicators, pathfinders, satellites, captives, or slaves. Bleeding from kimberlite in sand- to BB-size grains, the best indicators are certain rare blood -red pyrope garnets, pea-green chrome diopsides, and shiny black ilmenites. They may travel dozens or hundreds of miles before settling in sandbars, eskers, or beaches. Four or five grains culled from among billions may start the trail.>
From Barren Lands
An Epic Search for Diamonds in the North American Arctic
Kevin Krajick
typing by daughter Laila |
