Jon Goodwin, head of the Geoscience Information group, quizzes 4-H kids on a recent field trip to the Geological Survey. Goodwin nods, pointing to five specimens. The building blocks of rocks are minerals.
Kids crowd in to look.
Some are precipitated. They're solids separated from a solution, like the salts and other hard deposits left in a tea kettle after the water boils away. Each mineral has its own chemical composition. He picks up a chunk that reflects specks of light. You can identify a mineral by its form or crystal shape.