Here's why:

* Solid: Crystals are solid, meaning they have a definite shape and volume.

* Homogeneous: They are made of a single substance, meaning their composition is uniform throughout.

* Natural geometric form: Crystals form naturally with distinct, repeating patterns. These patterns are the result of the ordered arrangement of atoms or molecules within the crystal lattice.

Examples of crystals include:

* Quartz (silicon dioxide)

* Diamond (carbon)

* Halite (table salt)

* Calcite (calcium carbonate)