Purple SiO2 is Amethyst. It is caused by several things, irradiation by heat or light and iron impurities which sometimes combine with trace elements. I have seen it in the amethyst mines, red, black, brown, white, clear and different shades of purple. I've seen pieces as big as a car. Google the largest amethyst mine in the world. It is up in Ontario not far from Kakabeka falls. That is the second biggest waterfall east of the Mississippi on the North American continent. The flies are terrible but it was still fun.