Micro-Numismatic Quiz #15 Answered.
Insider2
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Sorry, no prizes 
I am going to be posting micrographs of anything that can be found on coins. Some will be easy and some will not. The correct answer is whatever I say it is 
but feel free to disagree (giving your reason) so all of us can discuss it. Images will be posted in this thread at my discretion - several each week. I will usually post the answers late the next day after the quiz was posted.
In order to make this FUN for beginners. I'll ask the "experts" to PLEASE not guess what any of the images are until the next day. Otherwise, folks like (fill in the blank) will get all of them immediately - That's NO FUN. If no correct answer is given by the next day, the "experts" should PLEASE respond. The answer for each image will usually be posted in the afternoon when I add another image.
The Rules:
1.Anyone can guess. That's because even an incorrect guess can open further discussion as to why it is incorrect.
2.The BEST guess is in two parts:
A. What characteristic the image shows.
B. What did you see in the image that led to your guess.
The Question: There is a change of color in the field of this Trade dollar. What could that indicate?
The Image:
Comments
altered surface
not sure what was used
The part that looks “off” is the lighter color.
Looks like it was all brownish and then something happened to cause the lighter “stains.”
WAG Alert.
So... someone tried to clean (due to all the darkness still around the edges of any relief, i.e., stars, rim, twig, etc.) the coin of the brown (thus the lighter color) and then it started to re-tone (brown within the lighter area).
I see a section of a slightly worn Trade Dollar obverse with what appears to be a stain from something it came in contact with while in circulation. The two smaller stains to the left and above left of the larger stains look like organic. Maybe a little mold?
Crude oil stain then went thru a brief fire?
it looks like it might have been puttied to cover up a rough patch. The differences in toning/color were my red flag.
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Looks like dark toning was cleaned off the field, leaving a light area, then something applied either to cover up surface defects or the light area (the brown amoeba).
I know nothing about the how and why, but I suspect putty or some type of filler. The way the blemishes disappear and the pattern of surface wear changes entirely. The depth of the contact marks LOOKS different, either from alterations or toning.
In conclusion, my best guess, repairs were made, to cover a chop mark or damage.
There are two sides of a coin, I love these quizzes, but would like to see the whole coin and yet that would take away the suspense... @Insider2 keep it up you rascal
Karate chop chop mark chop suey chop salad go chopping get to the choppa!
Chop
https://www.autismforums.com/media/albums/acrylic-colors-by-rocco.291/
Putty, or removed stain/gunk.
Many answers so far sound quite plausible. Is it possible this coin was struck on a planchet made from a poorly mixed alloy and the darker area is a copper richer area?
Worry is the interest you pay on a debt you may not owe.
"Paper money eventually returns to its intrinsic value---zero."----Voltaire
"Everything you say should be true, but not everything true should be said."----Voltaire
Spot removal?
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@Aspie_Rocco said: "There are two sides of a coin, I love these quizzes, but would like to see the whole coin and yet that would take away the suspense... @Insider2 keep it up you rascal
Chop Mark.
There are three sides. I only post what is necessary to answer the question so I don't honor requests to see the other sides. It is not necessary.
This is a repaired chop. First clue is the discoloration and the fact that the coin is a Trade dollar. Ron the design can be harder to detect. I once had an argument with a TPGS authenticator/finalizer about a T$ with a repaired chop on the eagle. It was plain as day to me and he refused to look at the coin under my scope. It went out as a straight-graded AU!
It was probably bought back decades ago.
@Insider2 Thanks for this.
What kind of fillers were used?
the other ide would have helped detect the repair. I also agree with aspie in both sides entire coin will put things in better perspective.
I was going to say your pic shows machine doubling near knee.
"Repaired chopmark"
Those were simpler times, when a repaired coin was more desirable than an honest coin with a chop. Unless it was super rare, I assume it would be more desireble these days with a chop than a repaired chop?
I remember when chop marks were considered to be damage and coins with them sold for a discount.
Worry is the interest you pay on a debt you may not owe.
"Paper money eventually returns to its intrinsic value---zero."----Voltaire
"Everything you say should be true, but not everything true should be said."----Voltaire
That's interesting! I didn't know they filled in chopmarks... Very cool thread indeed!
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The next clue is the fact that the discolored area is uncharacteristically free of contact marks when compared with the surrounding field.
Keeper of the VAM Catalog • Professional Coin Imaging • Prime Number Set • World Coins in Early America • British Trade Dollars • Variety Attribution