Good one Bjay,
Looks to be sedimentary - when I first looked at the image looked to be a compressed sediment with an above medium concentration or iron oxide. But that should have broken down in the HCl
Appears to be a claystone from the image. Pretty sure HCl doesnt attack or destroy the clay structure depending on concentration/minerals etc.
Is the rock brittle? Shale is also a fine grained compacted clay silt or mud sedimentary rock - is there any layering?
Nice find by the way