Mining Processes

Diamonds are recovered in two fundamentally different ways, and the difference follows the deposit. Pipe mining extracts diamonds from the kimberlite pipes where they formed. Alluvial mining recovers the stones that water, sea or wind carried away from those pipes into secondary beds. One works the place of origin; the other works everywhere the diamonds were taken.

Pipe mining

Inside a kimberlite pipe the rock changes with depth, and miners read those layers by colour. Near the surface lies the weathered yellow ground; below it the blue ground; and deeper still the unweathered hard ground. Past a certain depth, open-pit working gives way to underground mining.

Layer	Depth	Character
Yellow ground	10–30 m	Weathered, yellow kimberlite
Blue ground	30–500 m	Fresher, blue kimberlite
Hard ground	below 500 m	Unweathered, grey kimberlite

Alluvial mining

The earliest miners worked with picks and shovels and sifted the gravel in sieves. The principle has not changed; only the machinery has grown. Two old methods show how cleverly the diamond’s own weight was put to work.

The batea

The first alluvial technique was the batea — the pan. Swirled in a circular motion, the dense diamond separated itself from the lighter dust and settled to the bottom. It is the simplest method there is, and small-scale miners without the means for modern equipment still use it today.

The Long Tom

Borrowed from Australian gold fields and brought to South Africa’s diamond sector, the Long Tom both washed and roughly sorted. It is an inclined trough fitted with small wooden bars — riffles — nailed across it end to end. Gravel was tipped in at the top and flushed down with water; the heavier material lodged against the riffles and went straight to the sorting table.

At scale

In larger operations, earth-moving equipment feeds the alluvium through coarse sieving and a rotary trommel before it reaches a large washing pan. What remains then passes through concentrate sieving, a picking table and a grease table to recover the diamonds.

Deposits worked as dry diggings use much the same machinery; what changes is the way the ground is reached. Where alluvial diamonds have come to rest on dry land — often coastal tertiary beds — the stones tend to be heavy and sound, having survived several journeys, which makes such diggings especially worth the effort.