Recrystallization
Chert is an example of recystallization. Millions of radiolarian skeletons fall to the sea floor in the abyssal plain and are buried under sediment. Pressure due to burial causes the silica in the skeletons to recrystallize and chert is formed.
Another classic example of recrystallization is aragonite in shells recrystallizing into the more stable form of calcite.
Recrystallization is merely a reorganization of the original material that is being fossilized. It is not being molecularly changed, just reshaped and (often) resized (enlarged). This helps to make the fossil it will become to be sturdier and last throughout the years. Recrystallization only occurs when there are hard parts that are being fossilized (such as bones from vertebrates or shells from bivalves).
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"Recrystallized Trilobite." Ocean Fossils. Web. 2 Mar 2011. <http://www.cnsm.csulb.edu/departments/geology/people/bperry/Sedimentary%20Rocks%20Tour/fossils.htm>.
