Silica is the name given to a group of minerals composed of silicon and oxygen, the two most abundant elements in the earth's crust. Silica is found commonly in the crystalline state and rarely in an amorphous state. Silica exists in seven different crystalline forms with the three main forms being quartz (by far the most common), tridymite and cristobalite. Quartz is the second most common mineral in the earth's crust.
RCS refers to crystalline silica particles that, when inhaled, are small enough to penetrate the nose and upper respiratory system and reach deep into the lungs where oxygen – carbon dioxide transfer takes place. Because crystalline silica, as quartz, is a major constituent of all rocks, minerals and soils, practically all dust clouds contain some percentage of RCS.