A type of mineral in the chalcedony family of quartz. Usually agates are multicolored, showing parallel stripes of different colors, but sometimes random-looking patterns occur -- an example is "moss agate" which has several shades of green together and looks like moss growing on the ground until you get close to it. The most common agates are blue, green, and brown, but it comes in many colors naturally (and is also dyed to resemble other opaque stones).