They are still actively exploited in many ways, from the most primitive to the highly sophisticated. 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. 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.
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.