Danny Turnbull, age 8, of San Diego, Calif. for his question:

WHAT IS QUARTZ?

Quartz is a common mineral that can be found in many rocks. You'll be able to spot it easily because it looks like pieces of broken glass. It can be found in sandstone, where calcite or mica holds it together.

Quartz is the hardest of all common minerals. Only very rare minerals such as diamonds, topaz and corundum are harder. Quartz is not easily changed by moisture or by weather conditions.

There are many varieties of quartz. It can be found as six sided crystals or in one mass. Pure quartz, often called rock crystal, is sometimes cut for gems. Amethyst is a rare, clear, colored quartz. Quartz is also used in making sandpaper and grindstones.