Almost all gems are minerals, but only a very few minerals are gems. To be a gem, a mineral must be beautiful, rare, and durable. When a gem is rare, it is hard to find. Some gems are rarer than others. 

Most rough gems are crystals. Their shapes depend on their chemical structures. Aquamarine and emeralds come in hexagonal (six-sided) shapes, while diamonds and spinel are often octahedrons, for example. Not all rough gems are crystals—lapis lazuli is rock and amber is fossilized resin. 

Gem crystals can be cut and polished into shiny gems. When a gem is cut, it is referred to as a fashioned gem. Skillful fashioning can show off a gem’s luster or color. 

• A carat is the unit of measure used to weigh gems 
• Gems are found in certain places around the world. Brazil, India, Australia, and Madagascar are important sources of various gems.
• Some gems come from a famous mine or have a rich history, which can also make them more valuable.