Turquoise is an opaque, blue-to-green mineral that is rare and valuable. It is perhaps the most ancient stone. Jewelry and object of ornamentation have been found in Egyptian, Chinese and Aztec historic sites. Turquoise forms inside the Earth within host rocks under high temperature and pressure, making it as hard as glass. It is made mostly of copper and aluminum and comes in a range of different shades of blue, blue-green and brownish-green.