I assume the OP is talking about cut diamonds . . . Uncut diamonds are virtually impossible to find legally and no one but an expert could tell you if they have any value or not - since everything depends not only on the typical things like color and clarity, but more importantly, what the uncut diamonds could be cut into (which is itself an art).
So dealing with cut diamonds, I agree that just because the diamonds may have been mined in South or Southern Africa doesn’t mean there will be a deal on them or that they would be cheaper there than elsewhere. Any diamonds of any value (non-industrial) are sent to Antwerp (declining) or to Asia to be cut, and by the time they make it back to South Africa, they’re priced the same as anywhere.
Diamonds, like gold, have a global market value (wholesale and retail) which is independent of local variations, and anyone who sells below the market value is costing themselves money. Ask yourself why someone would do that for someone they don’t know and will never see again?
If I were in the market for diamonds, I would look to a local or US-based internet vendor who I would have recourse against if the diamond turned out not to be entirely as advertised. The difference in value between a VVS diamond and a VS or an SI diamond is substantial, and only a gemologist can certify which it is. Same with cut, color, etc. So some level of trust is required.
I tend to parcel out my trust carefully, and African diamond merchants aren’t on the list.
