From some second season episode.

I purchased this online.

This was actually given to me as a gift from a friend who lives on the east coast. It's from an episode in the second season, probably one where someone was threatening to shut down (or dig up) the caves under Smallville. The button not only features the text "Save the Cave", but also has a drawing of one of the main ideographs from the cave. I'm no archeologist, but I think this is the one from the rock where Clark places the key, and is given all kinds of info on the Kryptonian language.