You will come to a point in the library stage roughly halfway through the game where you will encounter a teleportation demon in a room with 2 goat heads along with several small monkey demons. Kill the monkey demons and then shoot the teleportation demon with your hot boner just as he is about to leap and attack you. It may take practice, but you will eventually knock him down to the floor. Quickly, jump on the demon by running next to him and hitting X. Keep hitting X as each time will give you 3-4 white gems. You can do this all the way up to the limit of 999 gems. If the demon breaks free just repeat the process until you get 999 gems. This glitch makes the Las Vegas achievement (getting all red gem upgrades) very easy.