As noted in the first post on this thread (which is where the "official guide" actually is), the amethyst is the gem for shopping, not referrals; it's the emerald that's based on a certain number of referrals' successfully completing an offer. As to what, exactly, is required for each gem, that's a mystery. There's a way to find out, but we can't tell you! (There's a hint, though, in the official guide as to how you can get additional information.... )
With regard to Mabutu's Secret, please submit a ticket via the SUPPORT tab at the top of your TreasureTrooper page.