The bottom three look heat toned. The top one might be as well, but could just have been in a Tupperware container with an egg for a while.
Toning is silver sulfide. One way or another, sulfur has been introduced to the silver under forced conditions, and you're seeing the result. When that sulfur is introduced more gradually - under conditions we're more likely to call "natural," things like the circles on the last Kennedy don't happen. That one looks to have been chemically stripped and poorly rinsed, leading to areas where the toning didn't "take" when forced upon it so strongly. These are some badly abused coins.
They came out of a box I searched yesterday so I'm not out any $$. They are going back to the dump bank tomorrow.
No it is NOT! It's the sulfides produced from heating them and subsequently reacting with the coin metal.
I'm not a fan of chemically altered coins...but to each his own. I read an article about a last who bought several rainbow toned quarters on eBay and took them to a coin grader. Not one was authentic and chemically induced. Usually, when natural the toning can be vivid but still remains subtle in its distinction.