Here's why:

* Elements are pure substances made up of only one type of atom (e.g., gold, oxygen, carbon).

* Compounds are substances made up of two or more different elements chemically bonded together (e.g., water, salt, carbon dioxide).

Crystals are solid materials with a specific, repeating three-dimensional arrangement of atoms or molecules. This structure is what gives crystals their characteristic shape and properties.

Examples of crystals:

* Diamond is a crystal made of pure carbon atoms. While diamond is made of an element, the arrangement of those atoms gives it its crystal structure.

* Quartz is a crystal made of silicon and oxygen atoms bonded together in a specific pattern. It is a compound, but it also has a crystalline structure.

Therefore, "crystal" refers to a physical structure, not a chemical composition. It can be made of elements, compounds, or even mixtures of elements and compounds.