Blue diamonds are so rare that few people have ever actually laid eyes on a vivid specimen; yet London-based jeweler Alisa Moussaieff (+44.20.7290.1536, www.moussaieff.co.uk) gazes at them daily, thanks to her unprecedented cache of these elusive stones. Among her treasures is a 39.19-carat rough blue diamond that she acquired for $8.8 million and intends to cut as a 9-carat, pear-shaped stone. "These diamonds are so scarce," she says, "that when one comes up for sale, you have to grab it." She had to move quickly to snap up the half-dozen sizable blue diamonds that she added to her exceptional collection last year, and she does not believe that they will remain in her possession for long—even during difficult economic times. "Historically, unusual diamonds have always been a good investment," she says. "And that’s not changing."