10 Francs Rhinoceros, ruthenium

Ruthenium is one of the rarest platinum-group metals, produced almost entirely as a byproduct of platinum and palladium refining in South Africa and Russia. Rwanda's use of it here is a collector marketing exercise rather than a monetary one — the face value is notional, the metal itself the point. Very few sovereign mints have worked with ruthenium at all, owing to its extreme brittleness and the difficulty of cold-working it into struck blanks.