Keilim, Chapter 12, Mishnah 7. Before we come to the words of the Mishnah, we need one basic principle: a coin is not a keli. A keli is something that performs a function, that does work for a person. A coin does no work at all. It is a medium of exchange, a way of trading labor or merchandise for something else, and it carries a value that everyone agrees upon. Because it accomplishes nothing on its own, it does not enter the category of kelim, and therefore a coin is not receptive to tumah.
But what happens when a coin can no longer serve as money? Perhaps the government changed the currency and this coin is no longer accepted. Perhaps the coin itself wore down, and the silver in it lost so much of its weight that it can no longer pass for its face value. Such a coin has lost its identity as currency. If its owner now takes it and puts it to some other use, how do we treat it?
A Coin That Became a Piece of Jewelry
The Mishnah teaches: "Dinar shenifsal ve'hitkino litloto betzavar ketanah": a dinar that became invalid, and its owner prepared it to be hung around the neck of a young girl. He pierced it, drew a thread through the hole, and hung it on her as a charm or an ornament on a necklace. The Mishnah rules "tamei": this piece of metal can now become tamei. It is no longer a coin at all. It has become an article of jewelry, and jewelry is receptive to tumah.
A Coin That Became a Weight
The same applies in a second case: "Vechein sela shenifsal ve'hitkinah lihiyot shokel bah": a sela that became invalid as currency, which its owner set aside to serve as a weight for weighing out silver and gold. Since the coin is heavy enough to serve that purpose, and he has designated it for exactly that, the Mishnah rules "temei'ah". It has become a keli with a real function, an object that does work, and so it can receive tumah.
Which Invalid Coins May Be Kept at All?
Notice what these two rulings assume. If an invalid coin can be repurposed and become tamei, then evidently a person is permitted to hold on to it. Yet the Chachamim were concerned in many situations about keeping such a coin at all, out of fear that a person might pass it off illegally as good money, using it as counterfeit currency. So the Mishnah must now define when keeping a damaged coin is permitted.
The Mishnah asks: "Ad kamah tipasel vih'yeh rashai lekaymah": how far may a sela deteriorate, how much of its weight may it lose, and still be permitted to keep? The answer: "Ad shnei dinarin". He may keep it as long as its value stands at two dinarin, which is exactly half of a sela. At that point it corresponds precisely to a shekel, since a shekel is half a sela, and the coin has a clear and honest identity: it can simply be used as a shekel.
"Pachot mikan yachok": should the sela erode past the value of a shekel, the owner is obligated to slice it apart, so that it cannot be palmed off on anyone even at the lower rate of a shekel. The reverse case is judged identically: a coin whose remaining value sits somewhere above the level of a shekel yet has not held its full worth as a sela must likewise be cut, since we have no interest in letting it travel from hand to hand under the pretense of being a whole sela.
What emerges is a very tight permission. A ruined sela may be retained in only one situation: when what is left of it comes out to a shekel exactly, half of the sum it once carried, because then it functions truthfully as a shekel coin and deceives no one. In any other condition it has to be destroyed. And in each of the cases above, the moment such a coin is assigned a fresh job, whether it hangs as a decoration on a young girl or sits in the balance as a weight, it has departed from the realm of currency and joined the realm of kelim, where the laws of tumah take hold.