Next to oxygen, silicon is by far the most abundant constituent of the earth's crust, though never occurring alone. It is united with oxygen to form silica (Si02) or enters into the formation of more complex compounds.

1. Quartz

Quartz (Si02)is anhydrous silica in a crystalline state and is one of the most abundant of minerals. It belongs in the hexagonal system, and crystallizes in hexagonal prisms capped by six-sided pyramids, or in double six-sided pyramids, or in modifications of these forms. It is insoluble in acids, except hydrofluoric, and only very slowly soluble in boiling caustic alkalies.

Quartz has no cleavage and is very hard (H=7), scratching glass readily, while it cannot be scratched with a knife; the specific gravity (sp. gr.) is 2.6.

When pure and symmetrically crystallized, quartz is transparent, colourless, and lustrous (rock crystal), but it more commonly is found in dull masses. Many different colours are produced by minute quantities of foreign substances in the crystals.

2. Chalcedony

Chalcedony occurs in spheroidal or stalactitic masses, composed of more or less concentric shells. The chemical composition and behaviour of this mineral are the same as in quartz, but the specific gravity is somewhat lower, and the optical properties are different. Chalcedony has a waxy appearance, and is translucent or semi-opaque, and of various pale colours.

3. Flint And Chert

Flint And Chert are mixtures of hydrated and anhydrous silica. They occur in amorphous masses of neutral or dark colours, and are opaque, or somewhat translucent in thin pieces.