Many crystalline species are "massive" meaning that they have no true crystalline form or rarely display it. Because they are massive, they are broken up into convenient sizes and as a result, have sharp edges, dangerous to work with. These are best tumbled expertly so as to allow crystal work and harness the energy in a loving way. In some instances, massive stones are carved into specialty shapes so as better to deliver the energy in a specific way.
Some other crystals are broken or badly chipped when they are extracted from the Earth or shipped from the mining areas. Careful polishing restores the dignity and beauty the crystal had initially and makes the crystal useful once again!