Why does it seem like only mint state and au coins produce lustrous toning? Take an old silver coin, say like 1870s, and if it's a proof, ms, or au, it's most likely has very nice rainbow toning. But once it wears down, it usually does not do this. So...does the wearing down of the coin, and loss of mint luster do this? Or do the oils from humans handling the coin prevent rainbow toning from happening.
I beleive that the luster itself is major contributing factor for nice toning. If you look at PCGS photograde, you will generally see the highest graded coins are toned as well. The second image you posted is obviously a cleaned coin. Thats not to say coins without luster won't tone. Also, fingerprints and oils from your skin will actually promote toning.
Toning occurs when the coin is undisturbed. Think of it like the rust process of most basic metals, when left alone, they rust (natural process). If maintained, it slows down the rust process.
Detecto - read what you find here - http://www.cointalk.com/content/27-luster-guide-beginners.html Also make sure that you read what you find at the links I added at the bottom of that. Meanwhile, this will help explain to you why luster is so important to toning - http://www.cointalk.com/t12578/ But in very simple terms this is why. A coin with luster has a surface that looks like this - /\/\/\/\/\/\/\ A coin with no luster has a surface that looks like this - ______________________ The coin with the luster, exposes more of the surface to the air than the coin with no luster.
