Chalcedony is a translucent, usually grayish blue, variety of cryptocrystaline, or fine-grained, Quartz with a fibrous microstructure. Chalcedony has traditionally been defined as a fibrous cryptocrystaline variety of Quartz, but more recently it has been shown that much Chalcedony is actually a mixture of Quartz and Moganite, another silicamineral.
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 (often in rather wild patterns) 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 called simply ‘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 – not concentric deposition of the Chalcedony – and the material is Chalcedony, not true Agate.
Chalcedony is available from many sources worldwide.