well the problem with diamond is that its inherently worth very little. the price is regulated by one specific interest which holds a monopoly over the diamond market as a whole.
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Nope. Leviev broke that monopoly about 15 years ago. Now others (almost all of whom are frum) buy direct from Russia and own mines in Africa as well.
Cocaine is the best investment with heroin a close second. Contact your local branch of the Lucchese or Gambino crime family, or buy direct from Colombia and risk your life rather than turning 20-25% over to the mob.