The challenge has risen!  Yes, von Kossa is a popular and standard
stain for bone embedded in methylmethacrylate.  Thickness of a section
can be thin, such as thin with GMA embedding.  Have used it for
detection of calcium in paraffin (not on bone sections which have been
decalcified, obvious this won't work).  I developed my in a sunny window
and have also done it under a light with regular light bulb, amazing
to see the section turn black before your eyes. I think one could even
do this stain as a surface stain on thick, ground sections just don't etch
the surface of the section with any solvents or acids (mild decalcification)
it could give brilliant results with a toluidine blue counterstain.  Saw
a wonderful PMMA section of bone (5 um section) with von Kossa/T blue
combo once, spectacular!

There are some references for using this stain for bone histotechnics,
if people want them.  I liked the stain, but kept my glassware CLEAN
and used ultrapure water.

Gayle Callis