I did some of that man made rock,,, I did about 3 batches and every one of them turned out,,,,,,, man made. I could do cool shapes, but the texture is just not the same. I suppose if you used a ton of sand, and not as much shell it would be better, but then you dont have any buffering capacity, would suck even worst for the BB folk.
Hate to get serious,, but Brad diamonds are not rare,,,, but large clear colorless ones are!
(Im a Graduate Jeweler from Revere Academy of Jewelry Arts, so Im biased)