This 1896 gold coin survived a house fire, but was burned with vinyl melted to its surface. Some coins, like this example, can be carefully restored by an expert.
Many coin have been heat treated so as to tone the coin and hide contact marks or hairlines from being seen. With a little experience, an artificially toned coin can be discerned from a naturally toned coin.
Heat Treated-Burned Coins
Above is an example of an artificially toned Morgan Dollar. Heat was used to tone the coin. Here are a couple of things "stick" out like a sore thumb. The toning is not evenly dispersed and where the colorful toning meets the brown toning, the edge isn't sharp, but the colors flow into each other. A natural toned Morgan will not tone this way. Also, the coin has obvious wear and if this coin had toned naturally then the toning wouldn't be as brilliant, but more subdued.