Today in Science
Both rubies and sapphires are forms of the mineral corundum (aluminum oxide), with different trace elements dictating their color. In the case of rubies, which are red, the trace element is chromium. Sapphires, which can be many different colors ranging from blue and green to orange and yellow, get their hues from traces of iron, titanium, magnesium, and copper. Corundum gems also come in a pink-orange form that are known as padparadscha, which are both rare and valuable.
Sapphires and rubies, along with diamonds and emeralds, are considered the four precious gemstones, so called because of their rarity. (Amethysts, too, were once considered a precious gem; however, once large deposits were discovered in Brazil in the 19th century, they were downgraded to semi-precious and lost a considerable amount of their value.) Translucence and hardness also contribute to a classification as a precious gem. Sapphires and rubies score a 9 out of 10 on the Mohs scale of hardness.
Learn more about gems and minerals by checking out the Minerals, Crystals, and Gems: Stepping Stones to Inquiry tool (K-8).