A true crystal ball is cut and polished from a single mass of quartz large enough to yield a flawless sphere, a demanding lapidary process since natural quartz commonly contains internal fractures, inclusions or color zoning that ruin a sphere partway through grinding, and large clear material has historically come from sources including Madagascar and Brazil.
The far more common substitute
Because a genuine flawless quartz sphere of any size is expensive and slow to produce, the great majority of crystal balls sold and used are glass, molded or ground more quickly and cheaply, and a trained eye can usually tell the two apart by an internal bubble or a swirl pattern that natural quartz does not show. Beryl spheres, from the same mineral family as aquamarine, were historically prized in England as a scrying medium distinct from both quartz and glass.
Care, and what a crack means
Practitioners commonly keep a crystal ball wrapped in dark cloth between sessions, both for physical protection and, in many traditions, to keep it from other people's gaze or handling. There is no restriction on who may own or use one, and a cracked or chipped sphere generally loses whatever value it had, whether as a scrying tool or simply as a fine mineral specimen, and is retired or replaced rather than repaired.