I posted some treasure cards a while back. I still like the idea, especially as it seems like something perfect for sharing-- takes some work to make but then they're useful forever. So, today I ran across a public domain book with color plates of gems and greedily filled my purse. I made a page with labels and one without. Maybe your thieves know the names/values of gems-- give them the first, everyone else has to learn from experience. Pdf here.