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.