That's a special effect of the diamond marketplace where diamonds aren't really “precious materials” but authenticatable natural diamonds are precious collectibles. (That is, it is the provenance and not the material that is precious.).

But the dynamics are the same so long as the supply glut is in things interchangeable with what is precious (e.g., diamonds that, down to the De Beers microetching, are indistinguishable from the ones that people place value on.)