I've done a *ton* of marble magnets (which I call "Bubble Magnets"...they look like bubbles to me!).
Recently, I've found some large, clear pebbles at Dollar Tree stores for $1 a bag. They have a very slight irredescence to them, but not enough to keep the image underneath from being seen. I've also used Aileene's Glass & Bead glue and have had no problems with it.
I'll try to get a pic of my remaining magnets taken (most were Xmas gifts) and film developed to post here sometime.