Matching a pair of natural stones is the reason stud earrings cost more than the sum of their parts: for a matched pair you need two stones of the same size, the same tone and, in the case of the clearer material, a similar amount of visible internal character. Clear quartz with too many inclusions appears milky, and amethyst varies from pale lilac to near-black, so pairs are selected from a batch rather than picked at random.
The settings and posts are sterling silver, stamped and unplated. Rose quartz in particular softens in prolonged direct sunlight, so these are not earrings to keep on a sunny windowsill.
Amethyst, in this context, needs no caveat: the colour is natural. (Our citrine, by contrast, is heat-treated amethyst, and we say so on that product page.)