Appearance
Amorphous Silicon is a brown powder, and crystalline Silicon is a grey colour with a metallic lustre.

Source
Silicon makes up 25.7% of the Earth's crust by mass and is the second most abundant element (Oxygen is the first). It does not occur free in nature, but occurs mainly as the oxide and as silicates. The oxide includes Sand, Quartz, Rock Crystal, Amethyst, Agate, Flint and Opal. The silicate form includes Asbestos, Granite Hornblende, Feldspar, Clay and Mica.

Silicon is prepared commercially by electrolysis with Carbon electrodes of a mixture of Silica and Carbon. Silicon is used extensively in solid-state devices and for this, ultra-pure Silicon is required. This is prepared by thermal decomposition of ultra-pure Trichlorosilane.

Uses
Silicon is one of the most useful elements to mankind. Sand and Clay, which both contain Silicon, are used to make concrete and cement. Sand is a principal ingredient of Glass. Silicon is a component of Steel, and Silicon Carbides are used as abrasives and in Lasers. Silicon is used in pottery, enamels and high temperature materials.

Silicon is used extensively in micro-electronic devices. The Silicon is doped with Boron, Gallium, Phosphorus or Arsenic for use in transistors, solar cells, rectifiers and other instruments.

General Information
Silicon is relatively inert. It is attacked by halogens and dilute alkali. It is resistant to attack by acids except Hydrofluoric Acid.

The hydrolysis of Silicon organic chlorides produces Silicones. Silicones can range in appearance from liquids to hard, glasslike solids.

Some Silicates, such as Asbestos are carcinogenic. Workers such as miners and stonecutters, who are exposed to siliceous dust often develop a serious lung disease called Silicosis.