Spinel
Until fairly modern times, all red gemstones were referred to as "rubies". It was only relatively recently that the rarer ruby has been differentiated from the more common spinel. The two gemstones can be distinguished on the basis of hardness and density, namely that a ruby is slightly harder and denser than a spinel. The two stones can also be told apart by their optical properties: a true ruby is dichroic while a spinel is singly refractive. The rarity of the spinel in this case though, makes it the biggest, uncut spinel in the world, given that it has only been polished slightly, but it hasn't received a proper cut gemelogically speaking.
Read more about this topic: Black Prince's Ruby