The Cause of Blank Coins
However, if your blank coin has a reeded edge on it, you're dealing with something entirely different. Most likely, someone sheared the metal faces off the coin, or ground out the devices and polished the surface, because they way the U.S. Mint makes coins, it is impossible for a planchet to be blank and still have reeded edges, since the reeded edges are created at the same instant the faces are struck.
Almost Blank Coins
There are some known coin striking issues that can result in an almost blank planchet with reeded edges. If you see a faint, but very blurry or weak impression on the planchet, it might be a die adjustment strike, which is worth $20 to $40 depending on the coin type and depth of strike. Another possiblity for almost blank coins is a strike-through error, where something got in between the dies and the planchet surface, such as cloth or grease, when the coin was struck thereby obscuring or obliterating the devices (or parts of them.) The value of these varies based on the degree of obstruction and eye appeal, and can run from $2 to $40 depending on what it was struck through.
Blank Coins with No Rims
If your blank coin doesn't even have the upraised rim yet, it's technically a coin blank. If it is made of silver, it is almost impossible to authenticate, but if it's clad, an expert in error coins could probably authenticate it, in which case (if authentic) it's worth maybe $50 or more. Keep in mind that a genuine coin blank will have a rough, perhaps sharp edge, and the blank may be off-color or grainy looking, depending on what part of the coin blank processing phase it was in when it left the mint. Beware of nice, shiny looking coin blanks with no upraised rim and a smooth edge. These usually just counterfeit blanks made to deceive vending machines.
Get Expert Advice About Your Blank Coin
In all matters of this sort, it's best to get your blank coin looked at by an expert. You can find a local PNG Coin Dealer, or send the specimen to a specialist in minting errors for an opinion. As a third option, if you can post a photo in the Coin Collecting Forum, or describe it very carefully there, and perhaps we can provide some additional information.