Are ancient prutahs (collectable) muksah? I would love to show them to my Shabbat guests. They are around 2000 years old. Are they legal tender?
Money is not muktzah because it is legal tender - it is muktzah because it has no intrinsic use (See Mishna Berura 310:24). Money fits into the broader category of anything which is not a כלי, a vessel, which is not allowed to be carried on Shabbos (Rema O.C. 308:7). This would seemingly include your coin, which has no intrinsic use.
However, even something of this stature of muktzah can be designated for use before Shabbos (Mishna Berura 308:34).
Depending on how fragile or valuable the coin is, it could be muktzah machmas chesron kis, muktzah because it would not normally be handled due to the risk of ruining it.