What makes a gemstone rare and why are some gems rarer than others? The answer lies in gemstone geology. Some gems, such as emerald, require extraordinary geologic circumstances to form. Not only do different rock types need to meet under the right conditions and crystallize, certain rare elements often also need to be part of the mixture for the crystal to be the right color. Tune in as Wim Vertriest, supervisor of field gemology, covers the chemistry of Earth’s crust and the geologic criteria for forming rubies, sapphires and some of the world’s most valuable gems.