Where Found Agate Agate, like Chalcedony , is a variety of quartz gemstones composed of layers of quartz, sometimes of different colors. Agate usually occurs as rounded nodules or veins. The layers of quartz are often concentric. The composition of agate varies greatly, but silica is always predominant, usually with alumina and oxide of iron.
Agate comes in most colors.
There are several types of agate. Since agate is porous, it is often dyed to enhance its natural color. Often treated agate is sold as Black Onyx. Petrified wood is fossilized wood that has had its organic matter replaced by agate. Hardness Agate will chip and crack rather easily though it has a hardness rating of 7.