10 Dollars - Elizabeth II Black Ice

The "Black Ice" series from Cook Islands uses a galvanic treatment process applied after striking to achieve its characteristic darkened surfaces — a post-mint finish that polarizes collectors, some of whom question whether the chemical alteration compromises the coin's long-term surface stability. Cook Islands has issued commemorative silver under its own authority since the 1970s despite having no meaningful domestic circulation economy, with the New Zealand dollar serving all practical monetary functions on the island.