Planted Tank Guru
Join Date: Jan 2010
Location: Contra Costa CA
Petrified wood is fine in an aquarium.
Jade is good, too.
Both are inert.
To prove this test the GH, KH, pH and TDS of the water they are soaking in. If it has not changed, then these are good to go!
If any rocks come from near a road I would be very careful to wash in hot soapy water to get rid of any petroleum products on them, or possible pesticides.
Rocks from a known safe garden or other location with no pesticides I just rinse off the dirt and add them to the tank.
Rocks or wood from a wet location (river, pond) might carry some disease or parasite to the fish, so these will be cleaned much better.
Try the cottonwood (Poplar sp). I would think it will rot in the tank really fast, or else never sink. It is a very light wood. Not toxic, though. I have a dried chunk, not in a tank, and it is very light.