A mineral (or occasionally an organic mineral) to be called a gemstone, it must be beautiful in its appearance and colour. A gemstone has to be durable i. e. it should be hard enough to survive constant use and handling without getting scratched or damaged. Finally, it must be rare, because its scarcity makes it valuable.

Most of the gemstones are natural inorganic minerals with a fixed chemical composition and a regular internal crystal structure. A few gems like Pearl and Amber come from plants and animals and are known as organic gems.

Then there are some gems, which are synthetic. They do not have natural origin but are made in laboratories. They have very similar physical properties as the natural gems, and can be cut and polished to imitate the real thing. Sometimes these are used as a substitute in jewellery as they cost substantially less than the real gems.