What is quartz rock classification?
Quartz crystals are prefectly safe and so are amethyst in an aquarium They are inert and do not form compounds that can hurt fish. Stream pebbles which are used as gravel are often granite ( made of quartz and feldspar).
I dont think that the quartz itself would be the problem, but like Elliot said, the jagged edges could injure the betta. A good test for bettas is rubbing the object against some pantyhose and if it snags on the pantyhose at all, you know that it can do the same to the betta's fins.
They are safe. The unsafe materials are those which may release salts or minerals into the water, like coral, shells and some kind of stones.
Quarts and Amethyst is safe, just make sure they are not too sharp where they are a danger to the fish. However, amethyst is often related to high iron contents (where the purple comes from) and the base rock that is changing colors is most likely from this iron content found around the amethyst. I would certainly remove the non-amethyst base of the amethyst. It wouldn't hurt to do the same to any base on the other crystals. Also gypsum sometimes looks like a quartz and sometimes like broken glass but is not safe for aquariums as it is reactive in water. So no gypsum.
