Diamonds are found in kimberlite, a volcanic rock. Kimberlite is found in clusters of between three and 100 pipes. These pipes are generally carrot-shaped, with the bulk of the diamond-bearing ore near the surface. The closer the ore is to the surface, the easier the pipe is to mine, and the larger the diamonds tend to be.
However, diamond rich kimberlite pipes are difficult to find. The majority don’t contain any diamonds at all. Of those that do, only a small percentage are of production quality.
Testing for Diamond Presence
Once kimberlite has been found, tests are undertaken to see if there is a diamond presence. These tests first involve using water pressure to force kimberlite samples through a series of screens to separate the grains of sand and gravel by size.
The midsized samples, which are most likely to contain diamonds, are then placed on shaker tables that use water and vibration to separate them by weight or gravity. Next, the heavier samples are put through an electro-magnetic device. This device divides the samples with a high magnetic signature from the samples that are less magnetic.
A second electromagnetic device is used on the high magnetic samples to remove the ones with the very highest magnetic characteristics. The remaining samples then make two passes through a magstream device, after which only samples with very similar compositions remain. Technicians then examine the remaining samples under microscope for a diamond presence.
Once diamond elements are found, more terrain sampling is done in tighter patterns. Further samples are taken to define a mineral trail back to the original kimberlite pipe.
Once the pipe is located, two-inch core holes are drilled to depths of hundreds of metres in order to define the shape and overall size of the pipe. The solid core is recovered and processed to determine the number of carats of diamonds per tonne of kimberlite.
The kimberlite is first softly crushed to prevent damaging the diamonds. Then the material is inspected and subjected to caustic dissolution. Caustic dissolution employs a highly-concentrated salt solution and heat to separate the diamonds from the kimberlite. Everything other than diamonds and some indicator minerals simply melts away. The diamonds are then cut, polished and ready to sell.