Haunted Gems appear in the Kingdom Rush: Vengeance Elite Levels of the Cursed Bargain Mini-Campaign.
They are similar in appearance to the Blazing Gem tower, and operate in a similar manner. They require a short charge period during which the gems on the floor around it will glow in a circle, before the main gem is lit up. An activated gem will target up to three enemies at once within its range and a total of 420 magic damage to them, before cutting out. It can be used to kill Ghosts, which could be hard to deal with early on with just your towers. Cooldown is 15 seconds.