Linda McLardy, age 12, of Toronto, Ont., Canada, for her question:

What is feldspar?

The crust of the Earth is made from hundreds of different rock minerals. The most plentiful of these minerals is feldspar. Its main ingredient is silica, a compound of the elements silicon and oxygen. Feldspar also contains various amounts of aluminum. In addition to these basic ingredients, a deposit of feldspar may contain sodium, potassium of perhaps a variety of other rock making minerals.

There are wide differences in the Earth's many deposits of feldspar. There are feldspars of milky white and dirty gray, others of grayish green, blue or reddish brown.

Some feldspars are glassy crystals that rate as semiprecious stones. All the assorted feldspars are tough stones that rate among the hardest common minerals of the Earth's crust. Some are cut to make decorative building stones, and others are used to make pottery and fine ceramic glazes.