Could so many large, natural diamonds be found that they become cheap trinkets?
In the Beni Bousera Massif (the Beni Bousera Highlands) in Morocco, there are huge deposits of rock in which beautiful octahedral diamond crystals, as large as two centimetres across, comprised 15% by volume. There are some other similar rocks known around the world.
Unfortunately, in these particular rocks, high subsurface temperatures have, at some point in the past, changed the diamonds to plain old graphite -- ordinary pencil lead.
But what would happen if the melting ice caps in Greenland or Antarctica, as the edges retreated, one day revealed rocks with similarly vast deposits of similarly-sized diamond crystals, but, at the new location, not changed to graphite?
The world’s diamond markets would be instantly swamped. Prices would crash to a tiny fraction of those which presently prevail.
Current diamond mines operate with a diamond content of approximately .00001% to .0001% diamond. Imagine if a large source of 15% diamond, more than 100,000 times richer, became available!
Such incredibly rich diamond-bearing rocks definitely exist within the Earth, as shown by the Beni Bousera and other deposits, and graphitization is purely a matter of random chance. Apart from beneath the polar ice caps, such rocks may even exist hidden beneath dense vegetation or elsewhere, just waiting to be found!
Reference: Pearson, D. G., Davies, G. R., Nixon, P. H., & Milledge, H. J. (1989). Graphitized diamonds from a peridotite massif in Morocco and implications for anomalous diamond occurrences. Nature 338:60–62.