Quartz, one of the most abundant minerals in the Earth's crust, is composed of silicon and oxygen and in its pure state is colorless. When iron replaces some of the silicon atoms, the color is purple. Purple quartz is called amethyst and is the most prized gem variety of quartz. Amethyst is found in a variety of geologic settings, but most commmonly as crystals lining cavities in volcanic rock (geodes). The major sources for amethyst are Brazil and Uruguay. This modified rectangular cut amethyst from Brazil is 109.66cts.