Gemstones fall into two broad categories: precious and semi-precious. Despite their name, precious gemstones are not necessarily the rarest or most expensive stones but rather the most sought-after and valued overall. The most popular precious gemstones are known as the big four: sapphires, rubies, emeralds, and diamonds. Among the others are opals and pearls.
Practically all other gemstones not in that category are considered semi-precious. The most popular of these are agate, rose quartz, black onyx, and the 12 birthstones.
There are other ways to classify different gemstones, such as natural, synthetic, and imitation gemstones. Organic and non-organic designations are sometimes used for formations such as fossilized tree resin, better known as amber.