yPilot plant tests with a 25 kW Hicom mill have shown that the Hicom mill successfully liberates diamonds from kimberlite in the presence of significant quantities of barren dolomite and chert. The Hicom mill is a high-intensity tumbling mill which uses a nutating action to generate acceleration fields 40 to 50 times stronger than gravity, producing rapid and intense breakage of ores and other materials, It can be used either autogenously or with steel or ceramic grinding media. Open circuit autogenous Hicom grinding tests were completed on two separate samples of a diamond-bearing kimberlite containing a quantity of barren dolomite and chert. A specially modified grinding chamber with 40 mm diameter discharge ports allowed the rejection of large barren lumps of dolomite and chert while maintaining sufficient residence time to liberate the diamonds from the softer kimberlite. Diamond and ceramic tracers were added during each test to determine whether any damage might occur, and to obtain residence time distributions. It was shown that diamonds are not damaged in the mill and that almost all of the kimberlite was reduced to finer than 4 mm to achieve full liberation of valuable diamonds.