Semiconductor materials such as silicon, gallium arsenide, cadmium telluride or copper indium diselenide are used. The crystalline silicon solar cell is the most common one. Currently about 90% of all solar cells worldwide are made of silicon. It is the second most common chemical element on earth after oxygen, and thus has almost unlimited availability. Silicon does not exist as a chemical element, but exists primarily in the form of silicate sand, SiO2, silicon’s most common compound. Silicon can be found in minerals such as quartz and its corresponding varieties (amethyst, agate, jasper, etc).