Amethyst

Let it be known quite simply that the chemical structure for amethyst is SiO2 relatively speaking. It's found mainly in desert areas or those that are prone to have floods. Also, it's known from it's origination in pockets of volcanos. Perhaps some had long since gone inactive when they're doing the mining of stone. Regardless, amethyst comes in many sizes, types, and colors which range from clear to reddish and purple. Basically, it comes from the chrystalline structure of quartz which is clear like glass that is blown. Here in these cavities though, the sand mixes with earth which has traces of iron in it really. Without the iron inclusion in much smaller amounts than would otherwise matter, the amethyst might even be some other color. As the sand is heated and gets molten as under the volcano, you'll also notice it mixing in with water as well that's got turned into steam. This then is what promotes the two fold mixing action along with secondary oxygen inclusion. That alone will be enough molecularly speaking for the breaking of bonds. If the mixture when heated is bubbling much, that's all well and good and fine since you'll want that which creates the blips and ridge of the stone. It'll have one solid core down at the middle with much larger stone formations around much like a flower. Don't forget though not to stir it at all as it's left then alone. The lack of stirring action alone won't be enough to preclude oxygen mixing, though the bubbling action probably will more than likely. By far, it's probably the easiest stone to make though hard on the patience as made known then at the time.