Roman rock-crystal icosahedron (20-sided dice) used in fortune-telling. Each face has a Latin letter on it and a corresponding Roman numeral.
Such polyhedral dice were thrown to obtain a number, which was then matched with pre-prepared oracle answers in a divination handbook.
You got a source for that? This is something often posted, and any sources I've seen say that might be what it was for, but no one is sure. And that they predate Rome by a good amount of time, being found in Greece and Egypt.