25 Dollars - Charles III Selectium

Selectium is a proprietary alloy developed by the Scottsdale Mint combining gold with five platinum-group metals — platinum, palladium, rhodium, iridium, and ruthenium — plus silver. The Solomon Islands has no meaningful connection to this material; the island nation's mint authority is essentially a licensing arrangement used by private mints seeking a sovereign issuer for bullion and novelty products.

Rhodium and iridium are among the rarest elements in the earth's crust, which makes the alloy composition more interesting on paper than in practice at these weights.