Here's why:

* Crystalline Structure: Minerals are formed by a specific, repeating arrangement of atoms, called a crystal lattice. This internal order dictates the external shape of the mineral.

* Formation Process: Minerals form through various geological processes, often involving cooling, crystallization from magma or solutions, or precipitation. These processes favor the growth of crystals.

* Variety of Forms: While many minerals form as distinct, well-shaped crystals, others might appear in more irregular shapes like masses, grains, or fibers. However, they still possess the internal crystalline structure.

While crystals are the most common form, some minerals occur in other forms:

* Amorphous: Lacking a defined crystalline structure, like opal.

* Glassy: Quickly cooled molten rock, like obsidian.

* Fibrous: Long, thread-like structures, like asbestos.

Let me know if you'd like to know more about specific mineral forms!