Quartz crystals occur in a wide range of colors caused by natural mineral impurities within the crystal lattice. In its purest form, quartz is colorless and transparent (rock crystal), but it can also appear purple (amethyst), brown to black (smoky quartz), pink (rose quartz), green (prase), red, and many other shades. As the second most abundant mineral on Earth, quartz commonly forms large, well-developed crystals that are highly sought after by mineral collectors and admired for their natural beauty and display appeal.