while reading this thread, I had next to the keyboard a bunch of stone disc pendants with holes in the middle that I had just bought from bead and jewelry suppliers on ebay (to wear), among which was a clear quartz crystal disc
(the keyword to use while searching is "donut", although of course they are flatter than that implies -- you could also use the Chinese word "pi")
I gave it a try in a glass of water, but because of the way it's cut, it has a lensing effect and shows its edges too readily -- too bad, because otherwise it might have been a good substitute for a "chinese" coin through the cloth that the spectator could feel and drop ...
on a tangent, though, these mineral discs might make nice props in place of chinese coins or coins in general for various effects
maybe dip some opaque one on a string into a glass of water and have it come out clear
or use different ones instead of coins of different countries, so that international trade relations do not clutter the stage of the effect
btw, I liked Pete McCabe's idea above about something invisible in water that could collect bubbles to appear