Can human bones and/or teeth which have lain buried in the earth for several years become discolored from their time in their environment? This...

Bones and teeth are among the last parts of a corpse to
undergo decay, so they may lie in contact with vegetation and/or soil for a considerable
length of time. During that time, fungi and environmental acids can act on bone to break
it down and increase its porosity, and various minerals in the soil can migrate into the
bone in an ion-exchange process that replaces the calcium that is leaching out. A
forensic scientist with a good grasp of the minerals present in local soils can use this
information to make an estimate of how long bones have been buried. By contrast, bones
that are exposed at the surface of the soil tend to become bleached white by the sun,
and in this case calcium that leaches away is not replaced by other minerals, so bones
that are on top of the soil do not stain as readily. In this case the loss of calcium
and the bleaching effects can be used for time
estimates.

If you have access to a subscription service
such as Ebsco, href="http://www.ncbi.nlm.nih.gov/pubmed/19549030">this is a really
interesting article with a lot of details about how the calculations are done. If not,
the link will take you to the abstract, which is also worth a
look.