That's a good question. I'll do my best to respond.
Those things created by man - whether handaxes, Rosetta Stones, Parthenon marbles, gold coin can be termed artifacts. Whether its an item of art is usually determined by professionals in that discipine. "I just found a stupid looking stone" says one person, "Oh, my God", says another -- "you just found a hand axe and proof that the bronze age culture was in this location."
Land, slaves and animals are not man-made, but that doesn't mean that they weren't sought after commodities. This element can open up the debate to regions where I'd prefer not to tred.
Right now, I'd prefer to continue to discuss ownership of (man-made) property. I've firmly believe and I've stated so many times that the Parthenon marbles belong to Greece -- I sincerely believe that the British claim on the marbles is (legally) very, very weak. Their claim on the Rosetta stone doesn't seem to me to be that strong, either. And I'm judging the British not by today's values, but by their own values at the time. You see, if the report of 1816 claimed that Lord Elgin did no wrong doing, and in fact praised him for his efforts, then I would have to say: "Oh, well, then was then, now is now." But that wasn't the case. The British in 1816 knew what Elgin did was thievery and stated so. By those standards, by the standards of 1816, the fact that the British knew the items to be stolen and have not done anything about it, is very puzzling.
But here's another hypothetical case that I would have no idea how to judge. Suppose a viking ship is discovered in international waters, off the coast of Greenland by an American team. Which Governments would have the legal claim to the ship: Greenland, Iceland, Denmark, Norway or USA? I wouldn't have the foggiest idea.