Do you have hard water, or are you keeping fish that prefer hard water and alkaline pH ?
I ask because many petrified wood specimens have some calcium in them, which won't cause any issues to speak of in the type of tank I mentioned, but might if you are going with soft/acidic water.
The possibly volcanic type of rock could also be lime [ calcium ] based. Do the acid test on both so you know.
Vinegar works best on rocks with high lime content, if you boil the vinegar first it will work better when it's hot.
Pickle vinegar, @7% acetic acid, works better than regular vinegar, which is 5% acetic acid. If you happen to have some CLR around, it works better than any vinegar will, and rinses off in clean water as easily as vinegar does. It has a different type of acid in it, one that dissolves calcium much faster, so it will show even tiny amounts of calcium in rocks.
If you see any sign of fizzing or bubbling, there is calcium in the rock, and it's not suitable to use in a soft or acidic water tank. But in hard or alkaline pH, it's not a big issue, unless it's nearly all calcium, like marble would be, and even then, depends what you are keeping. Snails would like it, most shrimp would, and hard water fish would too.