Nope got to disagree. His celt is grainy but polished. Mozarkite is a chert like you said, never seen one ground. It would be too hard. The celt has been polished by work on a grindstone by hand. You can polish granite by working through grits. It like Tjm said not be granite but IMO it's not a chert.
Fyi there are chert Celt like pieces, but they were worked by a billet or hammerstone and could be made flatter and sharper but more fragile on impact.