Agatized coral from Tampa Beach Florida U.S.A.

Agatized corals are fossils formed when silica from groundwater replaces old buried corals (as opposed to typical calcite-replaced fossil corals). They consist of blank fossils filled with various forms of quartz (another name for silica), mainly chalcedony, and in some cases both chalcedony and rock crystal quartz (quartz that is clear with large visible crystals capped by crystal faces). The term 'agatized' refers to the variety of translucent chalcedony called 'agate' which has very small crystals (micro- or cryptocrystalline) and is characterized by striations and rich coloration, properties common to agates worldwide.