The rock is beige, porous and has cavities. It was found in a mountain stream in Southern Alberta, Canada.
closed as off-topic by Leukocyte, arkaia, trond hansen, Fred, Jan Doggen Aug 30 at 13:26
This question appears to be off-topic. The users who voted to close gave this specific reason:
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I would go with tufa - freshwatter CaCO3 deposit. Check if reacts to 10% HCl
I agree with Imorgh, that it is almost certainly a bit of freshwater limestone. It contains the botryoidal open-space depositional texture, and other irregular 'flowstone' textures that one associates with carbonate deposition around roots, leaf litter and other organic remains. Over time the organics oxidize and disappear, leaving just the porous carbonate coating seen in the photo. It is probably a recent rock (in geological terms), and relatively soft. In the longer time scale such rocks usually erode into carbonate sand, or are cemented up with more carbonate (or rarely silica) filling in the cavities.