A gemstone called a gem, fine gem, jewel, precious stone, or semi-precious stone is a piece of mineral crystal that is in cut and polished form, is used to make jewelry or other adornments. However the certain rocks such as lapis lazuli and opal and occasionally organic materials that are not minerals such as amber and pearl are used for jewelry and are often considered to be gemstones as well most gemstones are hard, but some soft minerals are used in jewelry because of their luster or physical properties that have aesthetic value.
Apart from jewelry from earliest antiquity engraved gems and hardstone carvings such as cups were major luxury art forms in modern times gemstones are identified by a gemologist who describe gems and their characteristics using technical terminology specific to the field of gemology.
Gemstones are classified into different groups, species, and varieties, for example, ruby is the red variety of species corundum, while any other color of corundum is considered as sapphire. Other examples are the emerald, aquamarine, red beryl and morganite which are the varieties of the minerals species beryl.
Gems are characterized in the terms of refractive index, dispersion, specific gravity, hardness and lustre.