How diamonds are sorted into industrial quality diamonds and gemstones
Diamond sorting is a two-fold process. First, the ore that a mining operation yields has to be sifted, so that the diamonds are sorted from the other materials. Next, the judgment of trained and skilled individuals is required to separate gem quality stones from industrial grade stones.
Sifting diamonds from dirt
This mechanical sorting process relies primarily on the high density of diamonds. An older but still effective method is to use a washing pan, because the surface of a diamond resists being wetted by water but readily adheres to grease.
The washing pan forces heavy minerals like diamonds to the bottom and waste to the top. Crushed ore is mixed with a muddy water suspension, called ‘puddle’, and the mass is stirred by angled rotating blades in the circular washing pan. Heavier minerals settle to the bottom and are pushed toward an exit point, while lighter waste rises to the top and overflows as a separate stream of material. Wet gravel washes across three inclined surfaces covered with beeswax and paraffin. Diamonds stick to the grease, while wetted waste minerals flow past.
More commonly, cones and cyclones use heavy-media separation, relying on the high density of diamonds. Diamond-bearing concentrate is mixed with a fluid with a density close to that of diamonds. The separation occurs by swirling the mixture at low and high velocities in cones and cyclones respectively. Cyclones are virtually 100% efficient at separating diamonds and similar dense minerals from the original ore.
With 99 percent of the waste in the ore removed, further separations may use either a grease table or an x-ray separator.
Skilled individuals manually sort the recovered diamonds by examining each piece through a loupe. Rough diamonds can be sorted into 3 distinct categories:
- Gem quality: only about 20% of all rough diamonds are good enough for jewelry
- Industrial quality: 80%, while of good quality, are reserved for industrial use
- Crushing-boart or boart: stones with the worst diamond quality are crushed to be used as diamond dust (used to cut and polish gem quality diamonds)
After the diamonds are sorted, they go through the marking and cutting process.
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