nyt: nyc gems

Not too far from the Diana Ross Playground at West 81st Street, we examined a huge outcrop of bedrock called mica schist. This is the rock that underlies most of Manhattan and provides the foundation for its skyscrapers. Using the "Cambridge Guide to Minerals, Rocks and Fossils," we determined that the schist contained the following minerals: flashing bits of muscovite, a white mica that gives almost all of Central Park's outcrops a glittering sheen; and biotite, a black mica that gives the rocks their dark color. There were also flecks of pinkish feldspar. But most interestingly, we found shining, millimeter-size grains of red garnet.