U.S. coins struck thru sanding discs are an extremely rare type of mint error. The obverse has a rough granular pattern across the entire surface due to a sanding disc being placed between the blank planchet and the obverse die before being struck.
A small group of mint state silver American Eagles (10-15 coins) were discovered struck thru a sanding disc and have sold in the $3,000 range.
This proof nickel struck thru a sanding disc is one of two known and are extremely rare on any proof coin.