2006-D Jefferson Nickel With Bright Coppery Patches.

I'm thinking some sort of corrosive environment caused it.

I would have to agree with the others on some sort of environmental damage, looks etched and lower than the rest of the coins surface but here is one to keep an eye out for in your future searching, thinking it is pretty rare though.

LINK https://www.error-ref.com/copper_fo...truck-in_e3/

I remember on a different thread, Silviosi said that coins could have a mottled look from being buried in garden dirt that contained mildly acidic horse nuggets. But I think that coin was quite dull, and did not have such a brightness to the copper.

Much of the copper areas on this coin have a slightly grainy surface. It could be consistent with corrosion, although I imagine that the alloy is mixed with powdered metal. In most places the copper runs straight through the devices without interruption, seamless with the gray metal. The only place it looks flaked is by States, where the devices remind me of a Zincoln that had the plating stripped to reveal the zinc.

I assume there are salts, acids, and bases that might selectively attack the nickel and leave the copper intact, but I have no idea what they might be. That might be interesting to know.

At a holding cost of five cents, I will tuck it into a paper pocket for the Box of Mysteries. Thanks, guys!

Ok, so how do I distinguish between environmental damage and improperly annealed metal?

Ok, I am clearing this one off my desk this evening. At one point, I did read something to the effect that an improperly annealed coin would not show any patches on the side, because the improper annealing happens at the metal sheet stage. I am not sure if that is accurate information or not, and none of the TPG examples had visible rims to verify that notion.