The December 17, 2012 issue of Coin World had an article describing the Mint's acknowledgement of the "white spots" problem for modern silver coins, especially the .999 American Silver Eagles. In the article it states,
"During her less then 18 months as the U.S. Mint's quality division chief, Stacy Kelley-Scherer has focused her attention on a problem that has plagued the American Silver Eagle silver bullion coin almost since its 1986 inception – spots. So far, a solution that prevents spotting from occurring has eluded Mint officials... Each year of the program, collectors and dealers of the American Eagle silver dollars have reported spots or blotches on the obverse and reverse, on all finishes – bullion, Proof and Uncirculated – and on coins from all Mints. The spotting is random and can appear as a single spot, multiple spots crossing the field and devices, or in large blotches or patches consuming significant portions of a coin's design."
The article illustrated spotted Silver Eagles that had been graded by Numismatic Guaranty Corp. (NGC). But the problem is not limited to NGC or any other grading service. At PCGS, we have received Silver Eagles in sealed Mint boxes (the 500 ounce green "monster boxes") and opened them to find coins that have already spotted. We have also graded spot-free coins, sent them to customers, and then had them returned to us months later after they had developed spots. There seems to be no rhyme or reason as to why some coins spot and some don't. But it is clearly something that is happening at the U.S. Mint.
The problem is also not limited to Silver Eagles or coins from the U.S. Mint. We have seen the same spotting problem on modern U.S. silver commemoratives and modern coins from other Mints such as Canada, China, and Australia. Our feeling is that it has something to do with the .999 silver composition, as the earlier pre-1965 90% silver coins seldom spot. However, it could also have something to do with the way the planchets are prepared or washed. We are not sure of the cause of the spotting, and apparently neither are the Mints of the world.
When initially grading modern silver issues, PCGS will deduct for spots that are already evident. If coins spot after they are graded by PCGS, they are not covered by the terms of the PCGS grading guarantee. We anticipate that a two-tier market (spot-free and spotted) will develop, much as it has in the past 10 years or so in the U.S. generic gold market.