Diamond Deposit Types

As the ancient continents drifted apart and reassembled in new arrangements, the kimberlite pipes that carried diamonds upward were scattered across the globe. Today seven countries account for some 80% of world production: Australia, Zaire, Botswana, South Africa, Russia, Namibia and Angola.

Primary deposits

A primary deposit is a diamond found where it arrived — the kimberlite chimney itself, the original vehicle that conveyed the crystal from the depths to the surface. Mine a primary deposit and you are working the rock that the diamond travelled in, undisturbed since the eruption that placed it there.

Secondary deposits

Over millions of years, weather breaks kimberlite down. As the rock decomposes, the diamonds locked inside it are released and carried away by water, sea or wind — fluvial, marine and aeolian transport. Wherever they come to rest, they form a secondary deposit.

Here the diamond’s own density does the sorting. With a specific gravity of 3.52, a diamond is markedly heavier than the sand and gravel around it, so it tends to settle out where a current slackens. Stones recovered this way are known as alluvial diamonds.

Deposit	Where the diamond sits	Name
Primary	In the kimberlite pipe of its origin	Pipe diamond
Secondary	Moved by water, sea or wind from the pipe	Alluvial diamond
Tertiary, quaternary…	Re-transported again from a secondary bed	Alluvial diamond

A diamond need not stop at a single move. A secondary deposit can itself be eroded and the stones shifted onward into tertiary, quaternary and further beds — each step a fresh journey, sorting the heaviest, soundest crystals out as it goes.