Tristan da Cunha is one of the few UK Overseas Territories not to have its own currency. The island does, however, have a Commemorative Coins Ordinance which enables the Governor (with the prior approval of the British Secretary of State for Foreign & Commonwealth Affairs) to "cause coins to be made and issued to commemorate notable events of importance to Tristan". Coins made and issued under the provisions of this law are legal tender on the island; they are not really likely to be used as such though, because their collectible and metal value far exceed their face value.
Some of these are bullion coins in the internationally popular one quarter of a troy ounce (1/4 oz) of gold format. | 
