Fossilized dinosaur bones come in a variety of forms, depending on how they have been petrified.  Most dinosaur bones have been petrified with calcium, which yields a stony appearance and texture.  Agatized bones are petrified with silica or quartz, giving them a colorful, glassy appearance.

       Upon death, bacteria and other living things soon digest most plant and animal remains.  Thus some of the organic elements of life are recycled to build new organisms.  Rare and special circumstances are necessary for any parts or traces of an organism to be preserved as a fossil.  Preservation requires that an organism or its remains be buried rapidly, be insulated from oxygen and decay producing organims, and remain buried and undisturbed.  Under even the best of conditions, the preservation of soft parts is exceptionally rare.  Usually only hard skeletal elements are preserved as fossils.

       Various processes may produce fossilization.  The longer a fossil remains buried in the earth, the more changes it may undergo.

       In order for an organism or its parts to become fossilized, it has to have expired; it must have had hard parts and must have been buried rapidly in the right kind of sediment.

       Sediment : An ancient deposit made up by the accumulation of small particles of clay, silt, sand, gravel, or organic matter.

       Interment : Most fossils are preserved as a result of being covered by sand, mud, or volcanic ash.            

       At some time after burial, fossilization begins.  Bones change.  Ground water seeps in and carries the mineral silica with it filling in cavities in bones in addition a process called replacement or mineralization.  Calcium and phosphate, the original molecules that bone are made of are leached out and silica takes their place. The bone become rocks.  Although the materials change, the structure remain. When you look at these fossils under magnification you can still see the canals for blood vessels (honeycomb look) and the minuscule structure of the bone itself.

       Permineralization : Bone is often preserved with the original cellular spaces filled with minerals.

       Replacement or mineralization : The process by which fossils, while buried in the ground, gradually accumulate inorganic minerals which infill and/or replace the original bone or tooth, making them very hard and dense.  After cellular spaces have been filled, cell walls often dissolve and are replaced with minerals leaving visible structure but little or no organic materials. 

Astrodon johnstoni: the Maryland State Dinosaur

http://www.mgs.md.gov/esic/fs/fs12.html

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