Gemstones have their own science as well, called gemology, which is a branch of mineralogy. But the concept of a gemstone is based on human value judgements. A gemstone is usually defined as a highly attractive and valuable piece of cut and polished mineral that is used in jewelry or other decorations. The science of gemology studies the special physical and optical properties of this subset of the mineral world.
However, this definition of a gemstone is not particularly scientific since it does not make much reference to the physical properties of gemstones. Attempts to remedy this shortcoming by referring to the unusual hardness and rarity of gemstones have not been very successful. Some gemstones, such as fluorite, are not especially hard. Also, rarity depends on what is being mined at any given time. Amethyst, the violet variety of quartz, was once quite rare; but today it is fairly abundant.