Question

Are diamonds brittle?

Answer 1

yes diamonds are brittle

Answer 2

Diamonds are brittle precisely because they are so hard!

Hardness is the ability of a material to resist plastic deformation. That is, its ability to resist scratching or denting. It is controlled by the arrangement of atoms within the material and by the strengths of chemical bonds between atoms. In order to change the shape of a material, it is necessary to break chemical bonds between its atoms; stronger bonds therefore make a harder material.

A useful definition of toughness (the opposite of brittleness) is the amount of energy a material can absorb before it fractures. A material which can absorb a lot of energy is very tough; a material which cannot is very brittle.

In metals, the mechanism by which energy is absorbed is plastic deformation. Breaking chemical bonds requires energy, and any energy used in this way is no longer available for fracture. This is why when you hit a piece of metal with a hammer it dents but does not crack.

Atoms in diamond are held together by extremely strong covalent bonds, making them extremely hard. Because of this, diamond has no way of absorbing energy. All of the energy supplied to it in an impact is available for cracking, and so diamonds fail in an extremely brittle manner.

(Note that this is nothing to do with whether or not the material is crystalline. Metals are soft, tough crystals. Diamonds are hard, brittle crystals.)