Crystals & Gemstones are wonderful conduits for the transfer of energy. Piezo Electricity (the production of an electrical current when a crystal such as Quartz is stimulated) gives total objectivity to the fact that crystals have and transmit energies.
Crystals change and evolve over time, just as humans do. If you closely examine your crystals that have multiple flaws with a magnifying device, you will certainly notice changes occurring over time. Amethyst is one stone that appears to be more significantly affected than other crystals. The more the stones are worked with energetically, the more quickly they will be altered.