A mineral is said to have cleavage if it splits along a plane when hit with a hammer. Therefore, this can be used to identify samples.

There are 4 different types of cleavage:-

(1) Perfect e.g. Dumortiorite

(2) Distinct e.g. Alabaster

(3) Indistinct e.g. Pyrite

(4) Nonexistent e.g. Limonite

If a mineral does not break along a surface related to it's crystal structure then it fractures leaving uneven surfaces.

Twinning

Twinning occurs when 2 or more crystals appear to be growing into each other. e.g. pyrite/aragonite