Gemstones have ornamental and aesthetic value because of their luster and sheen which they get after being polished and cut. Otherwise, in their natural form, these gemstones are simply rocks and minerals.
Diamonds are Carbon which is black but it shimmers and glitters, becomes highly pried and valued after processing.
Similarly, Ruby is mineral Corundum comprised of Aluminum Oxide. Ruby acquires its pinkish Red color from the mineral Chrome or Chromium.
Many other Sapphires are also Corundum or Aluminum Oxide and get their respective colors from the traces of colorful minerals. For example, Blue Sapphire has trace mixtures of Titanium and Iron.
Emeralds are precious gemstones of the Beryl family. Beryl comprises of minerals namely beryllium, aluminum, silicon and oxygen. Pure Beryl is colorless. Aquamarine is a semiprecious gemstone of Beryl family with traces of iron imparting color to it..