Enlarge ImageGreen gold. A complex geological process produced this sample of nickel sulfide. Credit: Marco Fiorentini, Science Those spare nickels in your pocket might not be there without the help of ancient volcanoes that blasted sulfur dioxide into the sky billions of years ago. The discovery solves a mystery that has dogged researchers for decades, says geochemist Edward Ripley of Indiana University, Bloomington, who was not affiliated with the study. The nickel in ore deposits is actually nickel sulfide, a compound that is rich in sulfur. The sulfur is "critically important," says geochemist Douglas Rumble of the Carnegie Institution...