earthstory:

Three interpenetrating pyrite cubes

Many crystals are not the single orderly arrangement that one might expect from looking at their perfect outer shapes. They can change the orientation of their crystal structure during growth, and this phenomenon is often invisible from the outside.This phenomenon is called twinning, and produces many amazing crystal shapes.

Rubies for example have fine lamellae visible inside them under the microscope, each recording a separate crystal that grew after a change in growth orientation. In this case, three twinned pyrite crystals grew together in a form known as interpenetrant twinning.

Loz

Image credit: Vassil