The first reason is that “crystal-clear”, natural quartz is extremely rare to find in nature, and so that makes them super expensive to purchase. Another reason, I choose to use human-made crystals, is that natural quartz crystals have imperfections in them, and that makes them very difficult to cut into a multifaceted stone. If you just get the ones that are human-made, they're not going to have those imperfections in them. https://fengshuiit.wordpress.com/2018/01/04/faceted-crystals/