Sandy Martin, age 14, of College Park, Georgia, for her question:

 What is Corundum?

If you wanted the most costly gem, you might well choose a corundum. This precious stone is a crystal mineral made from everyday oxygen and aluminum. When tinted with rich color, a fine corundum crystal could be more valuable than a glittering diamond. It may be an Oriental amethyst, tinted with violet or with the velvety purple of irises. Green corundum is Oriental emerald. There are also topaz and ruby corundums. The word Oriental is used for these precious gems to distinguish them from less valuable minerals that create gems of the same colors.

Glassy clear corundum is mined in Greece and Turkey and ground up for use as abrasives and polishing powers. The valuable colored gems are nuggets embedded in rocks or loose in river gravel. They are found, once in a while, in parts of India and Africa, in Russia and Malagasy. Traces of iron and titanium add the sapphire blues to corundum crystals. The rubies get their blushes from traces of chromium oxide. Other extra ingredients add the colors to topaz and emerald corundum gems.