A diamond is classified as a pure substance and a pure element. This is because a diamond is made entirely of carbon atoms arranged in a specific crystal structure, known as a diamond cubic structure. In a pure element, all atoms are the same type, and diamonds consist of only carbon without any other elements or compounds mixed in.
When we talk about substances, a pure substance consists of a single type of material, which can include elements and compounds. Since a diamond is made of only one type of atom—carbon—it fits the definition of a pure substance. Additionally, since it contains only carbon and no other materials, it is also considered a pure element.
On the other hand, homogeneous and heterogeneous mixtures consist of two or more different substances. A homogeneous mixture has a uniform composition throughout, whereas a heterogeneous mixture has components that can be distinguished. Diamonds do not fall into either of these categories since they are not mixtures but rather a single, pure form of carbon.