Cindy D. LaValley, age 10, of Lethbridge, Alberta, Canada for her question:

What makes rubies red?

Precious rubies are glassy, rosy red gems. They are costly because they are beautiful, durable and very, very rare. Yet the earth creates them from quite ordinary atoms. The basic material is called corundum, a very hard crystal made from blend of oxygen and aluminum. Sometimes a trace of this or that impurity gets mixed into the crystal. These impurities may color the basic corundum blue, green or red. Rubies are red because the corundum contains a trace of chromium oxide. This impurity is a chemical compound made from shiny metal chromium and ordinary oxygen.