Natural, as in what would commonly be referred to as a real diamond (yes, I realise they're they're chemically and structurally the same).

Edit: As I said in another reply, there's nothing illogical about it. If value is placed culturally and socially on natural diamonds, then they are valuable, regardless of if that value is "artificially" created via advertising of whatever. Lots of stuff is quite stupid if you look at it objectively, out of context. Doesn't mean it isn't true in context.