@jkallmeyer. This is a pretty common misconception. The coloration of a fossil (or sub-fossil) is controlled by the surrounding sediment. The rate at which sediment becomes incorporated depends on a number of things from the porosity of the object to the chemistry of infiltrating water. You can also have superficial color changes from chemical weathering. I tend to advise against using color as a tool for identifying fossils because of how variable it can be. Here are a few tricks for determining if a bone is a fossil:
- You can a sound test. If you tap two fossil bones together, it sounds similar to tapping two ceramic pieces together. This is because fossil bone is typically ‘permineralized’, which basically means it has been infilled with minerals. Bone that hasn’t been permineralized will have a dull sound, similar to tapping two pieces of wood together.
- You can do a weight test. Another consequence of permineralization is that the bone becomes much denser. So sometimes just feeling the weight in your hand is enough to tell if you’ve got a fossil bone.
- You can do a burn test. Light a match under your bone in question. If it smells like burning hair and leaves a scorch mark, then it’s probably not fossil. Modern bones have collagen, which is what gives the burning hair smell. In fossil bones, organics like collagen are typically degraded.
These aren’t definitive methods, but they hold true in most scenarios. The section of pig jaw I found is very light and from the cross-section where it’s broken you can see the black is just a surface feature. There is no way anyone could tell that from the pictures I posted though and, I will admit, a few people at the museum thought it was fossil at first.