A gem can be organic or inorganic, natural or human-made. They can be minerals, rocks, or anything that has been given monetary value. Only about 80 minerals are considered gemstones, and probably only about 30 are commonly found in jewelry stores. Examples of mineral gemstones include garnet, hematite, and diamond.

Some minerals are given a specific gem name which is different from its mineral name. For example beryl is the mineral name, while the green variety is called an emerald. Corundum is the mineral name for both sapphires (blue variety) and rubies (red variety).

Some minerals are not minerals because they are organic in nature. A pearl, although mainly calcium carbonate, is produced by a mollusk. Amber, or fossilized tree resin is also considered a gemstone. Other gemstones are really rocks, which are multiple minerals. An example is polished granite or obsidian. Finally, some gemstones aren’t technically minerals because they are amorphous materials or lacking a regular crystal structure. Water within the chemical make-up of opal, creates an unorganized crystal structure.