Many minerals that are crystalline don't form visible crystal faces because they form in spaces that are crowded with other minerals which deny them the open space needed to fully form their faces.
For the same reason, an ideal crystal shape, like those shown above, is almost never seen in mineral specimens. The faces of the crystal may be elongated or distorted but the angles formed between any two like faces are always the same.
Mineralogy uses seven crystal systems to describe how a crystal will grow. Although it may not be apparent in some of the systems, all are based on symmetrical relationships between the atoms and, subsequently, the faces of the crystal.
To find rudimentary information about the types of symmetries in the models shown above, follow the links below the illustrations.
We have provided several beginning references for those interested in pursuing the details of crystallography on our Books Resource page.