I know that 2009 was a lower mintage year for nickels. I found one with a grease filled die, and the mint mark isn't visible to the naked eye. Any idea of the value?
Hello and welcome to Cointalk BTW. This is what most of us here think about microscopes.. For any of your future posts please provide full pictures of the Obverse and reverse of the coin. Keep in mind that any minor issues usually have very little or absolutely no premium at all. Learning about the issue is more important than "What's it worth?"
Generally speaking, we need pictures of the whole coin to give opinions, but also in general, such minor grease filling adds no value to a coin. It might even diminish value, being essentially a poor strike.
A grease filled die is very common and really doesn’t add value. It’s either a Denver or a Philadelphia coin and the mintages for that year are very high for either choice. Welcome to CT.
It’s quite possible that’s not a grease filled die at all. Look at a dimensional overlay for the coin question. The metal could have flowed into the other die.
When a planchet is struck by the hammer die the metal flows into the recesses of both dies (hammer and anvil). If the device has a high relief the metal may fill the device leaving less metal for the reverse. A popular coin that has this issue is the Jefferson nickel where as the obverse device void in the die fills and doesn’t leave enough material for the Monticello mansion on the reverse leading to a poor reverse strike. The LWC is probably best example of poor metal flow. Mr Lincoln’s bust had a very high relief the results in a poor strike for the E in ONE and the C in CENT. Planchet thickness, die spacing, strike pressure, strike speed, and die condition will also influence the metal flow between the dies.