River and Ocean Diamonds
Most diamonds are found underground, in a host rock called Kimberlite. This was called after the town of Kimberly, in South Africa, the place where this diamondiferous rock was first identified in the late 19th century. Kimberlite is an igneous rock, usually dark in colour. It occurs in carrot-shaped vertical pipes in the Earth’s crust, formed between 150 and 450 kilometres below ground, and is carried to the surface by magma eruptions. The pipes range in age from about 50 million to 2 billion years old. Though there are thousands of Kimberlite pipes in the world, there are only about 30 which are economically viable to mine.
Typical mining involves digging away the top surface of lava, and going through the Kimberlite to look for diamonds. Sometimes, Nature has done the job for us, over an immense period of time. Aeons of weathering and erosion will wear down the host rock, and the diamonds are carried downstream. Sometimes, they even reach the sea.
Where a river which passes through a kimberlite source meets the sea is often a terrific place to look for diamonds, as millions of years of weathering will have carried a large quantity of diamonds down to the delta. An added bonus is that poor quality rough diamonds with many flaws will not have survived the journey, so the diamonds that remain are typically very high quality.
One such sea mining operation is in Namibia, where the Debmarine Namibia, joint venture between De Beers and the government of Namibia is searching for shallow marine deposits. It drills a foot or so below the surface of the seabed, and scoops diamonds up into the vessel. A special X-ray device sorts the diamonds from the rest of the material, and the residue is returned to the sea. So far it has yielded 16 million carats of rough diamonds, of far higher material (almost 95% gem-quality) than a normal mine.