Agate

• A semi-precious chalcedony formed as quartz fossils of a previous geological age. The colors of agate can be clouded, clear or banded.
  Source: LITHICS-Net, Glossary of Lithics Terminology
• Variety of chalcedony with alternating layers of chalcedony and opal.
  Source: Leet, L. Don. 1982. Physical Geology, 6th Edition. Englewood Cliffs, NJ: Prentice-Hall
• A kind of silica consisting mainly of chalcedony in variegated bands or other patterns; commonly occupying vugs in volcanic and other rocks. AGI
• A translucent cryptocrystalline variety of variegated chalcedony commonly mixed or alternating with opal and characterized by colors arranged in alternating stripes or bands, in irregular clouds, or in mosslike forms; occurs in virtually all colors, generally of low intensity, in vugs in volcanic rocks and cavities in some other rocks. CF: onyx moss agate.
  Source: Dictionary of Mining, Mineral, and Related Terms