It's the aggregate, the colorful chips, that distinguishes terrazzo from plain old cement and lends it an artful complexity. Shiny glass, iridescent mother-of-pearl and bright plastic—in addition to traditional marble—offer a range of colors and virtually unlimited combinations of chips and pigments. To control the look of the finished product, terrazzo installers start with standard mixes, established by the National Terrazzo and Mosaic Association, and then make samples for their clients. "People want to coordinate with other flooring such as carpet or tile or even stone," says Dave Roberson of the David Allen Company, the Ewells' installer. "We can pretty well match a granite to where you can hardly tell the difference." After finalizing the recipe, Roberson goes to great pains to maintain consistency in the field, weighing and packaging the pigment and mixing the aggregates at the shop. "Even then," he says, "everything affects the color: humidity, temperature, even the amount of water. It's really more of an art than a science."