The Gemara says that in regards to a case where someone stole a new coin, and it got old and turned black while in the posession of the thief, it is considered a significant change and the thief does not return the coin as is, but rather pays the original value of the coin when it was new.
The Gemara says that perhaps you would say that it is not a significant change, because it could be shined again and look new. To this the Gemara says, “מידע ידיע שיחמייהו” – Their age (or blackness) is known.
What does this mean?
בע”ה נ”ל לפרש that when you take an old blackened coin and shine it to look ‘new,’ no matter how much you shine it, it will still be obvious that this is not a new coin, but rather an old coin that was shined. That being the case, the value of a newly minted coin (in ‘mint’ condition) is higher than an old and dirty coin, and also higher than an old coin that has been shined to remove the effects of age. Thus, the ‘age is known’ – i.e. it is clear that it is still an old coin, despite the fact that it has been shined, and therefore one would have to pay the original value of what he stole and would not be able to give back the coin as is.