Howdie,
I found that I just can not find any more info on that thunderstone that Father Zantus uses in the very beginning of Burnt Offerings. Or on any other thunderstone out there or thunderstones in general.
Can someone point me to what I am missing?
For future reference (in case you aren't at a computer), thunderstones are described in the Core Rulebook on page 160 under Special Substances and Items and listed on page 158 under the same heading.
Well, now I am embarassed to not have found it myself... Thanks folks.