Discolored Ulysses Grant Dollar Coins

I was speaking of the Ulysses dollars in particular. They always seem more brown and oily.

I was curious about this so I checked my collection. I live in a low humidity region and I have bought all of my Presidential dollars directly from the mint. I do have some slight discoloration on some of my 2007 and 2008 issues, but my USG dollars look the same as they did on day one. I suppose if I lived in another region I would have different results.

In 2000 I picked up a roll of Sacagawea dollars from my bank, this was a mint roll. The coins on either end showed a beautiful rainbow toning. I saved both of these coins and spent the rest. When Santa brings me a camera (I hope) then I can take some pictures of these coins and post them, they are a deep golden brown with purple and some green.

Most post 2000 circulation dollars I get are colored like the van buren on the left, but I've got a couple that were darker and oily looking like the ulysses on the right, and both of the oily dark ones were Ulysses so I just was curious as to why that was. Someone probably just used olive oil on both of the Ulysses. It was just odd that the only two I got that looked like that were both Ulysses, especially since I got them in two different places, albeit the same town.