What Makes A 1998-D Quarter Black?

Environmental damage

I would carry it a while in your pocket until the tops of the devices turn silver with a red field in the background. Gives it that antiqued look. (take images before and after and I can use them for educational files)

Looks like an oxide coating that has been rubbed off of the high points. If a Cu-Ni quarter is heated to red hot in a propane flame, it will turn dull-black on both sides when removed. The edge will retain the copper color for the most part. Heating a pure nickel coin this way leaves a loose, uneven black coating. Heating a pure copper coin this way also results in a black coating, but it flakes off easily. My theory is that the surface of the Cu-Ni quarter is being oxidized into a copper-nickel oxide of some sort. Such oxides are used for porous electrodes in super capacitors. It might just be a single metal oxide that adheres well to Cu-Ni.

This pic is of a 2000 quarter, a 1959 penny, and a Buffalo nickel; all heated in the way described above.

to the Community, Tammielle!