So, would getting all those diamonds out of the ground and into the markets effectively kill the African mines?
Not necessarily. Gem quality diamonds come from kimberlite pipes coming up from the deep earth, as do garnets. The supply of diamonds from those hasn't expanded, and the shock diamonds seem to be visually different enough that the two types could be kept seperate, as long as DeBeers and all the jewerls can convince the public that one type is an expensive gem, and the other is a crappy rock.
If they fail at that, then diamond prices will indeed collapse to somewhere slightly above mining, processing, cutting, mounting, and retailing costs. Since Russia gets so much revenue from selling gem-quality diamonds from their other mines, I'm sure they gave this a lot of thought before revealing the new source.