Quartz is one of the purest minerals known. Nevertheless, tiny amounts of impurities have an important effect on this crystalline form of silicon dioxide. The delicately hued type of natural quartz known as amethyst, for example, owes its distinctive purple color to traces of iron compounds that are locked into its crystal lattice as it grows.
By focusing on another type of impurity, researchers have now developed a simple method of determining growth rates along different directions in a quartz crystal. In effect, "we can see just what the crystal looked like throughout its growth history," says geophysicist Phillip D. Ihinger of Yale University.