Guyanese diamonds of the Mazaruni

Guyana has produced diamonds since 1887 and every single carat of it has come out of river gravel. There is no working diamond mine here in the pipe-and-headframe sense. The whole industry is alluvial, and the reason is the climate: deep tropical weathering rots the rock down and the rivers then do the sorting, concentrating whatever is chemically and physically tough enough to survive, which is exactly gold and diamond.

The heart of it is the Mazaruni basin, in the Pacaraima country of western Guyana. Production peaked in the 1920s at over two hundred thousand carats a year out of that basin, and total recorded production has passed six million carats. Through the 1980s declared output ran between four and twelve thousand carats annually, of which around sixty per cent was gem quality, which is a high proportion by world standards and is why Guyanese stones have a reputation out of proportion to the tonnage.

The geology is genuinely unresolved and that is the interesting part. Western Guyana is Guiana Shield basement overlain by the Roraima Supergroup, a two-thousand-three-hundred-metre pile of sandstone, conglomerate and ash-fall tuff laid down between about 2.12 and 1.78 billion years ago. Nobody has convincingly located the primary source of the diamonds. Papers have been published under titles as blunt as deciphering the enigmatic origin of Guyana's diamonds. The stones are demonstrably there. Where they came from is an open question.

Mining is still done by hand and by suction dredge in the interior rivers, largely by small operators, days from a road. This is not a visitable locality in any casual sense, and it should not be treated as one. It is on the map because it is one of the last places on Earth where a significant diamond field is worked essentially the way diamond fields were worked a hundred years ago.