Diamond engagement rings drove second-quarter sales up 18 percent for Tiffany & Co., but the iconic jeweler thinks women who buy their own bijoux are the next growth market. It is investing $13 million in gem designer Temple St. Clair's new boutiques, which will debut later this year at Orange County, Calif.'s South Coast Plaza and New Jersey's The Short Hills Mall. Wealthy women ages 30 to 60 who buy for themselves tend to favor more gold and colored stones, and Temple St. Clair will cater to them with trinkets selling at price points from $500 to $35,000.