A limb cast is a psuedomorph where the original wood has been replaced by chalcedony. These pieces form when a tree was buried by ash. The wood is burned away by the hot ash and a “cast” is left. Minerals in the groundwater then permeated the cast, replacing the original organic matter and turning it to stone. The result is a rock that has the form of a tree limb. Some of these casts show the bark and wood textures. Pink and green limb casts are highly prized by collectors.