The precise identification of some of these precious stones is uncertain. Unless you're a geologist, the terminology of birthstones may be a bit confusing. So here's a quick primer on birthstone terms. Rocks are made up of one or more minerals. Most people also include clay, sand and limestones in the rock category.
Gems are a special subgroup of highly prized minerals.
Gems are usually clearer, rarer, and more beautiful than other minerals. Color, luster and hardness are also important qualities that can set gems apart from other minerals. But there's no hard and fast rule as to which minerals cut it as gems and which don't. It's mostly a matter of custom. Most gems can be divided into precious and semi-precious.