What do stones mean in the context of Mineralogy? Gemstones, semi-precious stones or tumbled stones are minerals which are crystalline in nature and always form in one of six types of regular shapes or patterns, such as the common Quartz Crystal which has a hexagonal (six-sided) structure. Some stones such as Turquoise, may not appear like the typical transparent crystal because their crystal-like structure is at a microscopic level.
Rocks differ from stones in that a rock like Granite or Sodalite is non-homogeneous, which means it is composed of different minerals in varying proportions, in no regular pattern. Any piece will be slightly different from any other piece. All crystals are stones, whether they are clear “gemstones” like Emerald or the more common form of the same mineral, Green Beryl. Rocks may contain crystals, and small rocks are sometimes called stones; but technically, stones are always a single kind of crystal and not a piece of rock. Rocks are often composed of several types of crystals in varying proportions, but in the case of Quartz crystal it is 100% pure crystal.
Gemstones are considered valuable because of their beauty and relative rarity, often because they are perfectly clear and reflect the light in pretty colors. A perfectly clear Ruby, for instance, is a very rare and valuable transparent gemstone specimen of a stone called Red Corundum. A Sapphire is another variety of Corundum, differing from a Ruby only by the tiny quantities of trace minerals which give it a blue or white coloration instead of red.
Amethyst and Citrine are “semi-precious” gem-stone varieties of the common Quartz crystal, which differ only by their coloration
Emerald and Aquamarine are two varieties of the mineral Beryl which differ only by trace mineral content and color.