Marvin Moose, age 10, of Gastonia, North Carolina, for his question:

What is corundum?

Oxygen is the earth's most abundant element and aluminum is the crust's most plentiful metal. These two common ingredients create a dazzling group of the most gorgeous gems. They are corundums, second in hardness only to the durable diamond. Sometimes they out do the diamond in beauty. Like all precious stones, the corundums are rare. And, sad to say, almost all of them are found on the far side of the globe.

The valuable corundums are transparent crystals colored with traces of impurities. A trace of chromium oxide adds glittering red to the corundum ruby. The serene corundum sapphire gets its blue from a few molecules of iron and titanium. Other corundum gems include the precious Oriental emerald, the Oriental amethyst and the Oriental topaz. These, plus less valuable forms of corundum, are found in India and Malagasy, in Africa and Russia, Turkey and Greece.