Need Help Identifying Rocks

[Pandemonium]

I recently picked up some flat rocks to make a hide for my axolotl. I wanted a second opinion identifying these rocks, and especially whether they are aquarium safe.

These three are all from the same pick pile. I believe they are all slate:

This one is larger, feels less hard, and does have some air bubbles that escape when submerged:

Cleaning all of these with a toothbrush has had coloration come off on the brush, but the stones aren't crumbling.

 

[Autumnsky]

Those look like slate.

 

[Pandemonium]

Those look like slate.

That's what I thought, the red one I was worried might be a mudstone because it's much softer. I'll leave that one out to be safe. The gray slates should be inert, right?

 

[Autumnsky]

Slate is fine, and shouldn't change your pH.

The red one seems more solid "looking" than shale. Hard to tell for me in a pic only. We see grey and red slate here in Colorado. Which has the tannish / ochre color like the stone in the pic.

You might just try adding that stone to a bucket of water and leave it to soak, and see it it starts flaking apart, or muddying the water, and not fit for the aquarium.