Agate, a semiprecious variety of quartz valued both for its beauty and for its hardness. Pieces of agate range from less than one-half inch (1.3 cm) to more than two feet (60 cm) across. Most forms of agate are banded, having numerous curved or irregular bands of varying widths and different colors. Fortification agate has angular bands that resemble the plan of a typical 18th-century fort. A variety of quartz called onyx is similar to banded agate, except that in onyx the bands are straight and parallel.
A form called moss agate contains dark, mosslike patterns on a milky or nearly transparent background. These patterns are caused by the presence of manganese oxide or other impurities. Other forms of agate have irregular patterns of colors.
All forms of agate take a high polish. Most of the agate used for ornamental purposes is artificially colored. Banded agate can be readily dyed in a variety of colors. Because the adjacent bands differ in their permeability to dye solutions, the dye penetrates unevenly, giving rise to marked differences in appearance between bands.
Because of its hardness (6.57 on the Mohs scale), agate is used for such things as instrument bearings and grinding mortars. Because of its beauty agate is used in jewelry, bookends, paperweights, and other ornaments.
A district in southern Brazil and northern Uruguay is a major source of agate; another is the area near Oldenburg in Germany. In the United States, agate is obtained from the Pacific Coast and Rocky Mountain states, and from Texas.