Diamonds are a girl's best friend — but only if they are natural.

That is the message mining companies and luxury jewelers are keen to instill in consumers as the diamond trade faces a growing challenge from the production of synthetic diamonds.

Global diamond jewelry sales are worth more than $72 billion a year, according to the World Diamond Council, and jewelers like Tiffany and diamond miners led by De Beers stress that a jewel mined at great expense, and often with great risk, is infinitely more valuable than one made by man.