ALCON, I'm not a nickel guy so a little help please. I Have a AU53 buff here and wondering if this is just toning or improperly mixed alloy? Regards.
Due to its generally streaky nature I believe it is improper alloy mix. This is especially common on the 1913 Buffalo Nickels if I am correct.
This one is a little streaky, I think this is a toning and improperly cleaned issue. Toning is the deterioration of the surfaces.
Yes @Sting 60 I concur it is the equivalent of a "woodie" on a nickel, caused by an incomplete mixing of the copper and nickel during the melt, and showing as parallel streaks due to the strip rolling process.
Sometimes refered to by Ron Pope as war paint... man I miss him. He was the man on buffalo's! But yes improperly mixed.
Also worth noting that laminations are very common on Buffalo Nickels for the same reason and generally add no premium. Here is my 1913-S Type 2 Buffalo with a lamination right side of the obverse: