I pretty much agree with what PlantedRich said about the wood. Most wood is just cellulose (sugars/starches chained together in a way that's hard for most critters to digest/break down) and lignin (I'm not as certain about this, but it's a hydrocarbon polymer). Both of those are pretty much inert/harmless. It's the sap, resins, and stuff in the bark/leaves that can be problematic.
Probably why we refer to it as driftwood (even though most of it (mopane, manzanita, bogwood, etc.) isn't), since wood that's been in water a long time has had all the bark beaten off, and all the resins and sap leached out. Some woods are perfectly safe off the tree, but if you don't know, it's better to get well weathered wood, or barring that, extensively soak (or boil) it yourself.
Scratch the wood, especially on the end grain, and see if you can smell anything ('woody' and 'earthy' smells are fine, you want to be wary of anything like pine, cedar, etc.). Soft wood is perfectly fine to use, it just won't last as long.
I recently tore down a tank that had been up for about a year, and had some pretty soft wood in it (I think it might have been some variety of palm, but not certain), and there was close to a 1/8" difference in the part below the substrate. Pretty much just from snails grazing and chewing on it for a year.
Anyways, I'm guessing the second piece is some sort of woody vine - ivy, wisteria, etc. not certain, but that's what it reminds me of with the shape, and most of those are fairly soft.
The rock looks really familiar, but I can't quite place it. At first I wanted to say some type of schist, but that stuff isn't flaky, it's really strong. Also wanted to say graphite, but I don't think that's right. Maybe I'll think of it later today or tomorrow. If I can remember what it's reminding me of, I'll post.