Ellis Gray, age 8, o£ Philadelphia, Pennsylvania, for his question:

What causes the color of a ruby?

A rich, red ruby may be more costly than a sparkling diamond. Its value depends upon the beauty of its red coloring. And this rosy blush is added by some very ordinary atoms. The silvery metal on a shiny car is chromium. When atoms of chromium team up with atoms of oxygen, they form molecules of chromium oxide. This chromium oxide is the stuff that adds the rich, red color to a precious ruby. And there is just the tiniest trace of it in the recipe. The gem is made almost entirely from ordinary aluminum and ordinary oxygen. The atoms of these two elements are tied together in a special molecule package, forming a hard crystal mineral called corundum.

The smidgeon of coloring material is called an impurity because it does not really belong in the corundum recipe. Molecules of oxygen and other metals add green or heavenly blue to the corundum. These accidental impurities add beauty and extra value to the gem stones.