I would describe it as "die wear" (but I think there's a more accurate numismatic term which hasn't come to mind). What happens is metal flow, over time, digs into the striking dies. You see it commonly near areas of high relief on a coin, in your case the bust itself. This metal flow and the resulting depression of the die causes these types of raised areas and change in luster on may coins from this time period and also other time periods. For instance, it also commonly occurs on the reverse of Jefferson Nickels near the dome, but also on the obverse, as was pointed out in another thread by Lehigh. Large cents also suffer from this phenomenon during the same period and earlier. Mercury dimes and Walker Halves also commonly show this as well.
I'd asked the same question elsewhere. I had assumed it was simply due to a die clash, with that area a clash of the eagle's wing on the reverse. It was further explained by a CBH expert JRocco as being a "halo". "A halo is the result of a heavy lapping or filing of the dies after a clash. This is just a result of metal removed from the die in this area that is commonly lapped after a strong clash. This "halo" effect is not too uncommonly seen in front of her face."
