You may have heard the terms proof coin and un-circulated coin, but what's the difference between these two? To understand the difference between a proof and un-circulated coin, let's first answer the question, "What is an un-circulated coin?"
Un-circulated means a coin has not had any wear, such as the wear a coin might experience when it is used in commerce. Handling a coin, as well as improperly storing a coin, can result in wear on the surface of the coin. This wear, even if very minor, will cause a coin to no longer grade un-circulated.
When coins are minted they often bump into each other and receive small nicks and abrasion marks during the production process. These marks also occur as coins are transported in large canvas bags. These marks, sometimes called "bag marks", are more noticeable on larger coins, such as half dollars and dollars. Typical "bag marks" do not keep a coin from grading un-circulated. However, they can be an indicator of how high of a grade the un-circulated coin might receive.
Current accepted grading standards provide for a range of un-circulated grades, from the grade of MS-60 to MS-70. MS60 would be a lower grade (yet still) un-circulated coin with normal bag marks for that type of coin. Anything below MS-60 would not be considered un-circulated. MS70 would be the perfect "ideal" coin. Some coins are rare in grades MS65 to MS70, and even unheard of in MS70 grade. (The attribute "MS" stands for "mint state".)
A newly minted proof coin is also un-circulated, however it is the way it is made that causes a difference in appearance and qualifies it as a "proof". To understand this, let's look at how coins are made. Coins are produced when two dies strike a blank piece of metal with tremendous force. One die is engraved with the front (obverse) design for the coin. The other die has the back (reverse) coin design on it.
A proof coin is made with a specially polished and treated die! By treating the die in a special way, the coins it produces have a different appearance. Modern technology allows the high points on the coin design to be acid treated (on the die). The background (field) design of the coin die is polished, resulting in a mirror-like look on the coin it strikes. This gives the finished coin a frosted look (frosting) on the raise parts of the design, with a mirror like finish on the background. This contrasting finish is often called "cameo". On some older coins a cameo appearance is quite rare. The attribute "CAM", when added to a coin's description, means cameo appearance. "DCAM" means deep cameo, and indicates the cameo appearance is strong and easy to observe.
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