It's a naturally occurring mineral made up of carbon.
In it carbon atoms are oriented in a crystal structure known as diamond cubic.
Explanation:
If suppose we have a solid substance and we try to change its shape but it resists its shape change and rather it gets either fracture or gets broken, such substances are said to be brittle.
In the case of a diamond, atoms are held together by very strong covalent bonds that explain its hardness.
So when the external force is applied, then because of high bond strength it does not absorb energy rather the energy is used for cracking it and thus diamond is said to be brittle.