(DL) http://pubs.usgs.gov/pp/0828/report.pdf Page 24 lists a chemical analysis of eight fuller's earth samples as follows:
...Silicone (SiO2) 52.4 to 61% Range
...Aluminum (Al2O2) 9 to 14.8%
...Iron (Fe2O3) 2.9 to 4.6%

...Magnesium (MgO) 1.7 to 9.5%
...Calcium (CaO) 1.0 to 4.6%
...Potassium (K2O) 0.6 to 1.1%
...Titanium (TiO) 0.4 to 0.75%
...Polonium (PO) 0.1 to 2.4%
as well as other trace elements.
...(DL) Since the comparison is so strong; I propose that locally mined fuller's earth was the basic formula for Golden period violin making for applying to wood outer surface as the ground before the varnish coats of famous violin makers best instruments. This coating, I beleive, is responsible for dampening the mid and upper overtones for a superior violin sound.
(DL):
Varnish Top Layer
Wood of the violin plate.