Here's a pair of indexes to finish your week. Index fossils are the kind of fossils you look for the hardest, because they more than others will tell you exactly where your rocks sit in the stream of geologic time. Similarly, index minerals are the sure-fire indicators of the pressure and temperature conditions that a metamorphic rock had been through. With a firm "P-T path," you can put another piece together in the bigger story of what the Earth was doing when that rock was being kneaded and squeezed long ago. Indeed, with the combination of fossil ages and mineral data you can constrain geologic history better than either tool alone will let you. Indexes . . . good stuff.