Shabbos, Chapter 6, Mishnah 7. In the previous mishnah we introduced the concept of perifah - a way of fastening a garment on Shabbos. Our mishnah comes to teach us which objects may be used as a button for this purpose.
The text of the mishnah:
"Poref al ha'even ve'al ha'egoz ve'al hamatbe'a, u'vilvad shelo tifrof lechatchilah b'Shabbos"
The mishnah lists three objects that a woman may use for fastening:
"al ha'even" - a stone.
"ve'al ha'egoz" - a nut.
"ve'al hamatbe'a" - a coin.
Even though a stone and a coin are ordinarily muktzeh, they may be used for the purpose of fastening.
The condition - "u'vilvad shelo tifrof lechatchilah b'Shabbos":
The permission depends on the object not being made into a button for the first time on Shabbos itself. If the stone or the coin was already set aside and served as a button from before Shabbos, it may be used as a button on Shabbos. But if it was not set aside and not explicitly designated for this before Shabbos, it may not be used on Shabbos. Once the object already functions as a button, using it on Shabbos is permitted.
In summary: In this mishnah we learned that a stone, a nut and a coin are fit to be used for fastening on Shabbos, even though a stone and a coin are muktzeh, provided that they were prepared and designated for this role from before Shabbos and are not made into a button for the first time on Shabbos.