Most of the sands which we find on the beaches and in other places are the ruins of rocks which have come apart, usually as the result of the action of water. A large part of the ocean bottom is made up of "sandstone" and the continual washing of the water over this causes particles to break away and float off, whereupon they are swept up upon the beaches by the waves.

Sands differ in color according to the rocks from which they are derived. In addition to the sands on the beaches, they occur very abundantly in many inland locations, which were formerly sea bottoms, and very extensively in the great deserts of the world.

Valuable metallic ores, such as those of gold, platinum, tin, copper and iron, often occur in the form of sand or mixed with that substance. Pure siliceous sands are very valuable for the manufacture of glass, for making mortar, filters, ameliorating dense clay soils, for making molds in founding and for many other purposes.

The silica, which is the principal ingredient of sand, as well as of nearly all the earthy minerals, is known as "rock crystal" in its naturally crystallized form. Colored of a delicate purple, these crystals are what we call "amethysts." Silica is also met with in the "carnelian" and we find it constituting jasper, agate, cat's-eye, onyx and opals. In the latter it is combined with water. Many natural waters present us with silica in a dissolved state, although it is not soluble in pure water. The resistance offered by silica to all impressions is exemplified in the case of "flint" which consists essentially of silica colored with some impurity.