What this means is that in essence they are the same mineral. What differentiates Agate from Chalcedony is not its chemical composition, but its distinctive concentric and angular multi-colored banding.
However, there are always exceptions, such as Green Moss Agate whose banding is replaced by what can be termed ‘Landscape.’ The very appealing nature of ‘Landscape Agate’ looks like a pastoral country scene has been painted within the gem; in fact the verdant green agate is due to very fine canal fissures in the host rock in which mineral bearing water oxidized while the agate was forming.
Agate is an extremely diverse gem type and has many varieties including: Moss Green Agate, Botswana Agate, Madagascar Agate, Ellensburg Blue Agate, Crazy Lace Agate, Fire Agate, Bull’s Eye Agate, Coral Agate, Green Agate and Blue Agate to name but a few. Within the Agates are also the black and white banded Onyx and the red to white banded Sardonyx, which have been given their own names dating from the tradition of using them in cameo agate jewelry.
An Agate in its purest mineral form would be slightly off white to greyish blue bands. However, its most common occurrences are from white, gray, brown, orange, red, yellow to black resulting from iron oxides and hydroxides. The most collectable coloured Agates, and are also the rarest, are deep green Agate and blue Agate.
Agate forms in spaces within host rocks, usually volcanic igneous rock, but before Agate can form inside one of these cavities there must be the incidence of silica, water and minerals such as manganese or iron. These impurities will eventually harden at varying times giving the agate bands the variant colours for which it is known.
Agate Gems Part II