Proof Coins and Sets
The dies for making modern proof coins are often treated with chemicals to make certain parts of the design take on a frosted appearance, with the polished fields taking on a mirror finish.
Several other methods have been used in the past to achieve this effect, including sand blasting the dies, and matte proofs.
Proof coins of the early 19th century even appear to be scratched, but it was part of the production process. The term “proof” refers to the process by which the coins are made and not to the condition of the coin.