Jeff Jenlink, age 9, of Jet, Okla., for his question:

HOW ARE SELENITE CRYSTALS FORMED?

Selenite crystals are transparent. In its yellowish white form it is called gypsum and it is an important mineral that is used to make plaster of Paris.

Large beds of gypsum and selenite are supposed to have been formed when water evaporated from solutions of the mineral. The mineral is so soft that it can be scratched with your fingernail.

Gypsum is a hydrate of calcium sulfate. A hydrate contains water in addition to its other substances. When heated, gypsum loses three fourths of its water. The process is called calcination, and the remaining fine white powder is called plaster of Paris.

The United States is the world's leading producer of gypsum with Canada in second place.