Here is a rock that was presented to me in lab this week that I found interesting. It is a sample of the New Creek Limestone Formation found in western Virginia; also known as the Coeymans in New Jersey. It is a Devonian aged light gray, fossiliferous limestone, with very coarse grained calcite crystals. It is the fossils in the sample that I found most interesting. For some reason or another I have always been partial to crinoids. It might be because that was the first fossil I found on my own. With that said, the crinoid stem fossil found in this New Creek sample has retained its shape remarkably well, and it has a lot to do with diagenetic process of replacement. At some point during the process of this sediment becoming a rock the structure of the crinoid fossil (possibly aragonite) was replaced by calcite which is more stable. This process is sometimes called calcitization. If you look on the right half of the first picture below, you can make out the radial grooves in the fossil.
Mechanical pencil tip for scale.