Agates are a type of semi-precious stone, usually found in the form of a banded chalcedony. They are often used for jewelry and decorative items, and can be found in a variety of colors. Agates are formed when silica deposits fill cavities in volcanic rock, and are often found in riverbeds or near shorelines. The term 'MARBLES' is often used to refer to small, round objects made from glass, clay, or stone, which are used in a variety of games.