Whites Spectrum XLT, Garrett AT Pro, Macro pinpointer, Garrett carrot pinpointer,
Lesch digger, Nel Tornado coil for ATPro, Garret ATMax with Nel Tornado coil
It is quite possible that the crystalization on the petrified wood would be sap. During the state of being petrified the wood sap is exchange with surrounding minerals, and crystalized.
I noticed this was your very first post - so, Welcome Aboard Mattb! You didn't list your state (or country) in your profile. So, you might consider jumping over to Sub-Forum: Select Your Area.... for information (i.e., clubs, hunts, finds, legends, maps, etc.) directly related to your state (or country).
The phenomenon of crystallization within petrified wood is not at all unusual. In some cases it is likely the result of internal or external sap. I do not believe it to be the case with many of the specimens I have collected, however.
It can still be an attractive phenomenon, regardless of how it was formed.