If the crystal is forming in a rock, it generally does not have the room to form faces or the classic crystal form. The external crystal form is an excellent way to identify minerals. There are 6 basic crystal systems , which give rise to 32 crystal classes. The basic systems are isometric, tetragonal, hexagonal, orthorhombic, monoclinic, and triclinic. Isometric crystals generally have symmetrical faces, and are characteristically cubes, octahedrons, dodecahedrons, etc.
Isometric crystals have three axes of symmetry that are all at right angles to each other.
The axes are all of equal length. An example of a mineral with crystals in the isometric system is pyrite, which can form into nearly perfect cubes. Tetragonal crystals are often long and slender, or can even look like a needle. The crystals in this system characteristically form 4-sided prisms, pyramids, and dipyramids. The third axis is either longer or shorter than the other two, and is at right angles to them.