Here's why:

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

* Formed in Nature: Crystals can form naturally through geological processes, like the cooling of magma or the precipitation of minerals from water.

* Particles arranged in a repeating pattern: The defining characteristic of a crystal is its internal structure. Atoms, ions, or molecules within a crystal are arranged in a highly ordered, three-dimensional repeating pattern called a crystal lattice.

Let me know if you have any other questions!