Well, most say it is fine and safe but I have also seen in the Ginko petrified forest in Vantage Washington that the minerals can actually be poisonous and not to put it in your mouth or lick your fingers after handling it. It could be the soil from there and not the petrified wood itself but I would make sure I washed it extremely well before placing it in a tank. Not all the minerals in petrified wood are quartz. They can be calcite, magnesium, potasium, carbonates, silicates and many others. Thats where all the different colors come from.
Personally I have used it but just be aware that it could have an adverse effect on your fish and the levels of minerals. I just used very small pieces that way most of the leechable elements should have been minimul from natural weathering, kind of like driftwood.