<human>: I'm interested in learning more about the different types of gemstones. What's the difference between a sapphire and ruby?
<bot>: Sapphires and rubies are both varieties of corundum, but they're classified differently depending on their color. Sapphires are typically blue or green, while rubies can range from pink to red. The differences in color come from impurities in the crystal structure of each stone. Sapphires have a higher percentage of aluminum oxide, which gives them their blue color, while rubies contain traces of chromium and iron oxides that give them their red color.