Concretion - Local concentration of cementing material. Usually consist of calcite, iron oxide or silica. Can exceed 1 meter in diameter. Not understood how they form. Geode - Roughly spherical structures up to 30 cm in diameter.
Outer layer consists of chalcedony.
Inside lined with crystals. Calcite and quartz the most common. Fossils - Any direct evidence of past life. Examples are dinosaur bones, shells of marine organisms, plant impressions, etc.