I have no idea. Seems they only cost $5. If they were made of something special, you'd think they'd cost more than that.
I've read in the past that they take all sorts of little doodad souvenirs up with them, including a lot of patches, to be distributed later to friends and VIPs. You know, so you can say "Hey, look at this, it's actually been in space!"
Of course, so has any random rock you might pick up, when you think about it. But I suppose it means something to have had your little doodad put into space by mankind.