Chalcedonies include many types of cryptocrystalline quartz gems and feature a number of different colors. Geologists can tell a chalcedony from the arrangement and structure of its crystals.

But let’s be a bit more specific. What gives gemstones colors, or rather, what changes the way they absorb and reflect light so that they come out in so many different, striking colors?

Its name derives from the medieval ... blue and yellow. A garnet's color depends on its chemical composition, according tothe International Gem Society. For instance, almandine garnets range ...