Geodes are not sources of fossils, but some were originally fossils. A geode is a nodule of stone having a cavity lined with crystals or minerals.
It originated as a hole: possibly a hole left in rock by a dissolved fossil, or it may have been the hole inside an existing fossil. Water rich in silica percolated through the rock, depositing layers of silica around the edges of the cavity. Sometimes, this process continued until the cavity was filled with layers of agate or crystalline quartz. Often it stopped before this stage was reached, and a crystal-lined, hollow, quartz "geode" was left in the cavity.
In a few areas of Indiana and Kentucky, Mississippian shales release geodes that are unmistakably of fossil origin, for they are shaped like high-spired snails, crinoid cups, brachiopods, and corals. However, they are giants. Crinoid stems may be as big as a man's wrist and brachiopods the size of oranges. The exterior is rough and cracked. It appears that the fossils had swollen to five or ten times their normal size. This must have happened while the silica was forming —perhaps it formed on the outside of the organism, pushing away the surrounding shale as it grew. The center was then filled with quartz crystals. No actual shelly material remains, only the obese casts of the fossils.
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