A crystalline material is a substance with a regular, repeating, 3-dimensional atomic structure. We divide crystalline materials into monocrystalline and polycrystalline materials.

The Substance on the Left is Amorphous Carbon which is very Soft and Brittle the Substance on the Right is Diamond, the Hardest Mineral Currently Known. Both Substances are Composed Solely of Carbon Atoms – the Arrangement of the Atoms Makes a Major Difference.

An illustration to clarify the difference between an amorphous substance and a crystalline one. On the left you can see carbon atoms which are arranged in a random way; there is no regular repeating three-dimensional array of atoms like on the right. Although both substances are composed solely out of carbon atoms this arrangement of atoms makes a huge difference. The substance on the left is amorphous carbon which is very soft and brittle while the substance on the right is diamond, the hardest mineral we know.