What ? Paid 25 USD for a rock ?

Kryptonite rock, Pro Arts Inc., USA
1977

 ... that's what my parents said when I saw this in an Antique store in Indiana. But mam, it's Kryptonite ! And it glows in the dark (still does, after 34 years...)

I particularly enjoyed some of the instructions that came with the booklet:

• Do not keep your Kryptonite Rock in the same room as one of your Super Hero comics
• Do not lend your Kryptonite Rock to anyone, you never know whose side they are on
• remember, Superman says, "it's terrific - it's fantastic - it's Kryptonastic !"

So yes, paid about 25 bucks for a stone with green paint on it, and it's worth every penny.