The price of a coin depends upon its scarcity and state of preservation or 'grade'. Whereas frequently encountered and well-circulated coins are readily available at a low price, rare pieces even of average quality can be very expensive. The grade of a coin, therefore, is a decisive factor in determining its worth. For this reason, collectors should place great importance upon keeping their coins in pristine condition. As a rule, modern coins minted during the 20th century are only collectible in the grades of brilliant uncirculated, uncirculated and extremely fine.
Not a grade, but rather the special production method of minting collector's coins is typified by the terms polished plate, mint gloss or proof. In the minting of these flawless coins, specific attention is paid to achieving an optimum contrast between the lustre of the field and the frosted appearance of the relief. These coins are struck with polished dies on polished blanks. Proof-quality coins must never be touched with bare hands, for should these coins become damaged, their worth is downgraded to that of average-circulated coins. It is, therefore, highly advisable to keep collector's pieces in their original, generally sturdy, protective packaging (e.g. plastic capsules).
The accompanying list of coin condition descriptions was kindly provided by Häberling Numismatik, Zurich.