I've had this rock in my yard since we moved in over a decade ago. Some of the larger flat rock (shale?) , I've had in my tank for a couple years with no perceivable I'll effects. The other marbled ones, I'm afraid might have more mineralization. Almost look like petrified... Something. Haven't tried vinegar or h2o2 yet as it's still wet from getting hosed off. All seem pretty hard though. Anyone have any idea what they might be or if they might be dangerous? Appreciate any insight!
No idea what the makeup of the darker part of that rock is. The lighter part looks like a vein of something... possibly quartz? (I figure odds are it's either quartz or calcite, but I think quartz is more likely). Try scratching it with a nail/knife point. If steel can scratch it, it's probably calcite. If it won't scratch, it's probably quartz.
Calcite will raise pH and hardness, quartz is inert.
The white part seems to be quartz then, unable to scratch it with a steel dental pick, so that's good. Black part scratches pretty easily. Will have to figure out what that part is... Good info lochaber, thank you.
Bottom right rock is Quartz. The white and glass in it is from mild crystallization. If you see any deep red in there, that would indicate a higher iron content and the red is oxidation (aka rust). Sometimes you might find the quartz crystals in pink (rose quartz) and is a bit more rare. I would think the quartz is aquarium safe since you can get quartz sand but depends on what else is mixed in (ground up quartz).
I think the rocks on the left are slate and not shale. Difference being slate is compressed more and harder. Shale is more brittle and not as uniform in texture (will have smaller rocks mixed in). Also safe for aquarium as you already know.
Last on in the upper rt. I'm not sure from the pic. You guess of fossilized wood sounds accurate.