COMMENTARY — Pipes susceptible to erosion

Kimberlites and lamproites were intruded through the Earth’s crust over a wide variety of ages. Not surprisingly, they have all experienced some degree of erosion.

In Botswana, a thick cover of windblown desert sand has protected the pipes, so that the impact of weathering and erosion has been minimal. By contrast, in West Africa, little remains of the original pipes. Around the famous diamond city of Kimberley in South Africa, there has been an estimated 1,500 metres of erosion.

Kimberlite, the world’s principal source of diamonds, is highly susceptible to the weathering effects of wind and rain, but the diamonds this igneous rock may or may not contain are not.

In the central and western part of the southern African sub-continent, about 3,000 kimberlite pipes and dykes (mostly 90-120 million years ago) are believed to have released 10 billion carats of diamonds. This has resulted in the formation of the world’s richest diamond field, the marine beach terraces along 1,000 km of the Atlantic coast of Namibia and South Africa. As with vein and other hard-rock gold deposits, the original source of diamonds may be found by retracing the track of alluvial placers upstream. The principal difference is that while gold, because of its heavy weight, tends to coalesce into pieces of increasing size, diamonds disperse and their grades tend to become more diluted.

Gem diamonds are effectively indestructible. Thus, they may be transported enormous distances over tens of thousands of millennia, resulting in them becoming widely distributed over huge areas, as has happened in southern Africa.

In Russia and the Canadian Arctic, glaciers have managed an even more efficient job of dispersion than wind and water, ensuring tremendous distribution. In normal circumstances, 25-35% of the diamonds contained in a kimberlite pipe are of gem quality. These will usually survive the threat of abrasion and mechanical destruction of long journeys, while their lower-grade cousins, the near gem and industrial stones, will not.

Any flaw, inclusion or other weakness in a diamond’s crystal lattice increases the chances of its destruction, the greater the distance it is transported by natural forces.

— From a 1993 report on diamonds by researchers at Yorkton Securities.