"You never know what’s inside a geode until you crack it open," says jewelry designer Kimberly McDonald (www.kimberlymcdonald.com; available at Bergdorf Goodman, 212.?753.7300). "Each rock is so distinctive; some sparkle, some bubble, and some appear like a rainbow of colors." Any visitor to a natural-history museum will recognize these rough rocks—black, gray, green, reddish-clay, and off-white—that, when sliced open, reveal internal cavities encrusted with crystal formations. But the miniature versions of these curiosities, which the New York–based McDonald discovered toward the end of 2006 while scouting stones for private clients, absolutely captivated her. "It’s unexpected to see an asteroid-looking rock set with precious diamonds and gold in a stylish design," rel="nofollow" observes McDonald, who develops the concepts for her pieces ($3,000–$95,000) according to the idiosyncrasies and unique characteristics of the individual stones.