20 Dollars - Elizabeth II Nautilus

The nautilus shell has been used as a natural geometry study since at least the Renaissance, though the oft-cited claim that its spiral follows the golden ratio is mathematically imprecise — it approximates logarithmic growth, but the ratio varies by species. Cook Islands has issued high-relief and shaped silver pieces under its currency authority for decades, leveraging a longstanding arrangement with New Zealand that allows the islands to produce legal tender without the infrastructure of an independent central bank.