Just thought I would post some pictures of these (Ryncholampas gouldii) echinoids that I cut/ground/sanded/polished into cross-sections to show the variety of the fossilization processes. The white one was found on the bank of the River in a limestone matrix. The other five were found in a different river bottom, all in close proximity to each other. Even though they all were in the same type of dark sediment/rock, they all have different combinations of materials/layers/etc. were all extremely dense and hard to cut/grind - like a granite/marble material. For anyone not familiar, one of the pictures shows the profile of a full intact specimen (a "before" picture). Another picture shows an "after" profile picture, showing how these were taken down to approximately 1/4 inch thickness. For what it's worth, the darker the material, the harder/more dense it is.