Where are formed and why are some hollow and some not

Crystal is a solid that is composed of atoms arranged in an orderly pattern. Most nonliving substances are made up of crystals. For example, metals and rocks consist of crystals, as do snowflakes, salt, and sugar.

Well-developed crystals have a distinctly regular shape as a result of their geometrically ordered arrangement of atoms. Such crystals have smooth, flat surfaces, which intersect to form sharp edges. These surfaces, called crystal faces, show definite symmetrical relationships. The faces of crystals of the same substance always meet at the same angle regardless of the shape and size of the crystals.

The scientific study of crystals is called crystallography. Crystallographers measure the angles between crystal faces and analyze the symmetrical arrangements of such surfaces. They also examine and identify the atomic structures of crystals with the aid of transmission electron microscopes and X-ray diffraction techniques.