Common, semiprecious silica mineral, a variety of chalcedony that occurs in bands of varying colour and transparency. Varieties are characterized by peculiarities in the shape and colour of the bands, which are seen in sections cut at right angles to the layers. Agate is found throughout the world, commonly in cavities in eruptive rocks and in geodes. Brazil and Uruguay are major producers of agates; they are also found in Oregon, Washington, Idaho, Montana, and other western U.S. states. Agate is essentially quartz. Much commercial agate is artificially dyed to make the naturally dull-gray stones more colourful.