This article is fascinating for showing how much larger than the stones themselves the diamond product is. The stones aren't the product at all. The product is public opinion about diamonds. The suppliers create public opinion through product placement and advertising, and control price both by controlling supply, and by marketing diamonds in such a way as to discourage resale, which has the potential of upsetting the price in a big way.
That is the diamond product. The stones themselves are inconsequential.
The article is from 1982. It doesn't seem like a whole lot has changed since then; but it would be fascinating to read of later developments in the story.