Gillian

Got this one today and came to the comments to add it since I didn’t see it but was clearly too slow. I can at least help with the logic behind this one, though! If black’s statement that the other two statements are true is correct, that would mean all three statements are true. Since we know at least one is false, we know black’s message is false. If the blue box’s message is telling the truth and the gems are in the blue box, we have the same problem with all the messages being true, so we know that neither the blue box nor the black box can be true, which means only white is telling the truth. If white’s message that the gems aren’t in that box HAS to be true, and blue’s message that the gems are in that box CAN’T be true, then the gems are definitively in the black box.