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Minerals also form other shapes which are not crystals. The mineral smithsonite forms rounded crusts on rocks and minerals. Pyrite may form smoothly rounded lumps called nodules.
If you find a mineral with perfectly flat surfaces, it is likely a crystal. Some minerals break cleanly when hit, leaving pieces with smooth surfaces that look like crystals.
These clean breaks are called "cleavages". Each mineral tends to break or cleave more easily in some directions than in others. The way it breaks is called its cleavage pattern. Often you can identify minerals by a combination of their crystal shape and their cleavage pattern.
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