Most of the diamond deposits first discovered were alluvial -- concentrations in streambed or riverbed sand and gravel. They are still actively exploited in many ways, from the most primitive to the highly sophisticated. The goal is relatively simple: to find a location where moving water has deposited diamonds in the bottom of a channel, possibly in a pocket or cleft. Because rivers meander and drainage can change, fossilizing a once active river, the search for alluvial diamonds requires some geological knowledge and a lot of luck. The process involves removing the overlying barren ground, digging up the bearing ground, extracting the diamonds, and, nowadays, restoring the landscape when finished.

In the most basic, individual operations, such as in Sierra Leone or Angola, the technology involves shovel and pan, with some hand sloshing to gravitate diamond to the bottom of the pan; the eye is the ultimate sorting device. Mom-and-pop operations in South Africa involve a small claim and utilize limited technology -- shovels, buckets, jury-rigged cranes powered by small vehicles, and the like -- to load a small washing pan. The concentrate is then sieved into several size ranges, and each fraction is dumped onto a picking table, where someone checks by eye for diamonds. In the bigger operations, as shown in the model, large earth-moving equipment transports the alluvium, and the processing approaches that of the primary mines -- coarse sieving, then rotary sieving in a trommel, before loading into a large washing pan. Final processing includes concentrate sieving, a picking table, and usually a grease table. Of course, no crushing is required, as nature has already released the diamonds from the pipe rock.

"independent" operation photo: Craig Chesek, courtesy U.S. Geological Survey Library, Reston, VA.
All other photos: George E. Harlow, American Museum of Natural History/p>