The following is reproduced courtesy of Michael Byers (member 88) of www.byersnc.com
There are approximately 15 known 1999 State Quarters struck on Experimental Planchets. All five states in the 1999 series (DE, PA, CT, GA and NJ) have been discovered.
These Experimental State Quarters have sold for as high as $10,000 each, depending on which state, the coin's condition and which type of experimental composition was used.
There are four known types of experimental compositions which have been discovered so far (November 2001) on 1999 State Quarters.
Type #1 - This type has the "color" of a Sacagawea Dollar and has a copper center core.
Type #2 - This type has the "color" of the Sacagawea Dollar but does not have a copper center core.
Type #3 - This type has a slight "green" color and has a copper center core.
Type #4 - This type has a slight "green" color but does not have a copper center core.
Here are a few "telltale" signs to determine if you have discovered a State Quarter on an EXPERIMENTAL PLANCHET in circulation:
WEIGHT - ALL examples discovered so far are OVERWEIGHT - varying from 5.9 grams to 6.3 grams.
SIZE - It will be slightly THICKER than a regular State Quarter, due to the heavier planchet.
COLOR - So far, the pieces discovered are either the same "color" as a Sacagawea Dollar, or they have a slight "green hue" to them.
LOOK - The edge is slightly rough and may have a higher rim around part of the edge.
EDGE - Some of these do NOT have a center COPPER CORE.
STRIKE - None discovered so far are proof-like in the fields.
REEDING - Some of these have incomplete reeding.
These coins were analyzed by scanning electron microscophy with energy-dispersive x-ray spectroscopy (SEM-EDX) to determine alloy composition. The predominant metal is copper, followed by zinc. There are also small percentages of manganese and nickel.
PCGS and NGC have both authenticated and certified these 1999 State Quarters as being struck on experimental planchets.
In a recent Freedom of Information Act (FOIA) request for copies of any reports and test results, the Department of the Treasury acknowledged that "the U.S. Mint conducted engineering and metallurgical tests as part of its development of an alloy for the Golden Dollar", but would not release any information.
Editor's note: It is unclear if these are truly experimental pieces or some sort of mint error. It is very possible that these were deliberately struck. We are listing them under mint errors as this sector of the collecting fraternity was the first to embrace these. We hope that further information into their true nature will appear in the future.