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Welcome to Just Answer. A pumice stone isn't poisonous. It is possibly a foreign body that may block him up. I would take him to your vet to have them take an x-ray to confirm that it is inside of him. At that point, then I would monitor him for signs of a blockage - if he starts to vomit, stop eating or have abdominal pain - then a blockage would be possible. There is a chance that if the stone was ate and it is small enough, it may pass.
How urgent is this? Would you recommend taking him in right now?
Ok. We can try to make him vomit it up.
Here is a link that takes you through the steps of inducing vomiting at home. http://www.petplace.com/dogs/how-to-induce-vomiting-emesis-in-dogs/page1.aspx
If he vomits it up - great. If he doesn't - it isn't an emergency to rush him in. I would just have your vet x-ray him tomorrow to see if it is there.