Silicon ions can have a charge of positive four, positive two or negative four. The charge depends on the element with which the silicon has bonded.
Silicon does not occur by itself in nature because silicon ions bond to other elements with opposing charges to form various compounds. For example, silicon dioxide is the most common compound in the Earth's crust. It comes in many different forms, including sand, quartz, amethyst, agate, jasper, flint and opal. Even silica gel, a common moisture absorber, is made from silicon dioxide. Silicon carbide is almost as hard as diamonds, and it is often used to polish softer materials.