1979 Washington Quarter Weight 5g

Your coin looks to match number 6 in the list of Dirty Dozen Damages. Even though there is no premium value to these, you might try to find a nice example of each.

1) discoloration - stains from coffee, or environmental damage from being buried, heated, etc.
2) scrapes over much of the coin - damage from sliding on pavement, a parking lot coin
3) coin bent or edges not round - it has been smashed with a hammer
4) coin blank on all or most of one side - someone sanded it down
5) mirrored lettering - a vise job, a coin squeezed against another in a vise
6) rough, pebbly surfaces - coin that received an acid bath
7) smooth rims, smaller diameter - has been trapped rolling inside a dryer, a " dryer coin"
8) clear mounds on coin - glue that has dried transparently
9) small indentations in the shape of the letter D - marks left by the impact of the reeded edge of another coin
10) large blisters - coin exposed to high heat, such as in a campfire
11) shapes, often letters or numbers, not indented or raised - Pareidolia (like animal shapes in a cloud)
12) a circular scrape just inside the rim - "ring of death" caused by a coin rolling machine

Don't despair! Error coins remain ready to find from circulation, but they are outnumbered by unusual looking coins that merely have been damaged. If you can imagine a way to change an undamaged coin into one like you see, that's probably exactly what happened to it. Changes to a coin after it leaves the mint's striking chamber are considered post mint damage, or PMD, and have no premium value.

Agreed,acid damage.
John1

Great pictures!

@nick10 wonder if we can help find links to images for your wonderful post.

very cool acid coin - I used to have one like that, but I gifted it

Thank you @Dearborn! I will keep it for myself.