The University of Chicago Press
Blue Bones (page 79)
Although Pleistocene bones from silt deposits in Beringia are themselves stained dark from organic chemicals and minerals (they range in shades from ivory to jet black), their surface is often covered with a dusting of brilliant blue. As our bison mummy dried, its gray surface also turned blue. This blue was the same color as the dust used to chalk billiard cues or to powder a mason's line--an incongruous hue on the remains of so old an animal.
The blue that forms on Beringian fossils is actually a mineral known as vivianite, an iron phosphate; whitish gray in its unoxidized condition, vivianite soon turns blue when exposed to air. This mineral occurs where organic remains of animals, low in iron but high in phosphates, are buried in damp silt that is relatively rich with iron but phosphate poor. Palynologists see vivianite layers in pond sediments (H. E. Wright, pers. comm.). Presumably, this situation arises from a similar combination of iron oxides (which precipitate out in standing water) and phosphate-rich organic chemicals from the remains of pond organisms. Such bands of vivianite are white when the pond is first cored, but like Pleistocene bones, they turn blue when exposed to air. Chinese potters used iron phosphates in their glazes to obtain the blue-green celadon which imitated jade.
Vivianite appears as an irregular dusting on many fossil bones I have collected. Sometimes this coating is so thin as to be barely visible; on other specimens it is a thick crust that totally masks the underlying bone. In the latter case all one sees is a blue form in the shape of a bone. Blue Babe was not only covered with a dusting of vivianite, but additional wartlike growths (fig. 2.26) of vivianite crystals occurred in clusters on the skin (the size of 0.5-1.0 cm). These blue warts were especially apparent on the head. When they were removed in the preservation process, the underlying skin showed pock-shaped erosion craters, undoubtedly the result of chemical breakdown of phosphorus-containing skin proteins, particularly collagen, in the underlying dermis.
After Blue Babe was mounted we restored his color by taking vivianite collected from other bones and dusting it over the skin. A thick mixture of vivianite and shellac was also dabbed into the craters remaining on the head skin to re-create the blue warts.