What makes one type of mineral more valuable, and why are gemstones so precious? Firstly, we need to state that the term "gemstone" in the strict geological science sense is used to define a mineral crystal. A mineral has to be a solid chemical compound, with a crystal lattice structure. Gemstones can be either natural or synthetic, and to be considered as precious, they have to be exceedingly rare. Diamonds, rubies, sapphires, and emeralds are precious stones.