Agate was named for the Achates River, Acate, Ragusa Province, Sicily, Italy by Theophrastus, a Greek philosopher and naturalist, who discovered the stone along the shore of the river Achates sometime between the 4th and 3rd centuries BC. This river is now called the Dirillo river and colorful Agates and other Chalcedonies can still be found there.
Agate is a distinctly bandedvariety of Chalcedony which is a variety of Quartz. Chalcedony is a translucentvariety of cryptocrystaline, or fine-grained, Quartz with a fibrous microstructure. Agate is characterised by its concentric (having the same center or axis) banding which may be fairly simple or wildly patterned and often brightly colored by many different types of impurities. Agate has many variety names based on colors, locations and patterns. Some of the most common varieties are Blue Lace Agate, Botswana Agate, Brecciated Agate, Crazy Lace Agate, Fairburn Agate, Fortification Agate, Laguna Agate, Lake Superior Agate, Mexican Lace Agate, Nipomo Agate, Pigeon Blood Agate and Plume Agate.
Chalcedony includes many subvarieties such as the many varieties of Agate, Binghamite, Bloodstone, Carnelian, Chrysoprase, Onyx, Pietersite, Petrified Wood, Sard and Sardonyx. When Chalcedony is concentrically banded it is called by the subvariety name Agate. When it is in flat layers or bands of black and white, it is called by the subvariety name Onyx. When it is in flat layers or bands of light and dark shades of reds, browns and white, it is called by the subvariety name Sardonyx.
Many non-banded forms of Chalcedony – such as Moss Agate, are often erroneously called ‘Agates’. True Agate is concentrically banded. Mottled and included Chalcedonies are more properly simply called ‘Chalcedony.’ Petrified Wood (agatized wood) is the name given to fossil wood where the replacement of the wood is by Chalcedony, but the banding in this case is due to the wood structure or growth rings – not concentric deposition of the Chalcedony – and the material is Chalcedony, not true Agate.