My first science was geology: as a kid I collected rocks. A key test in geology is the scratch test for hardness, with a set of ten index values known as the Moh's Scale -- and I had all but #10, as Mom would not lend me her engagement ring.
Diamonds have all sorts of amazing properties, and a report in PNAS describes one more. Nanodiamonds around 35nM in size can be used as fluorescent tags. This entry's title derives from one interesting property: they don't photobleach. Organic dyes exposed to light for an extended period bleach out, but not these diamonds.
You'll need a subscription to view the article (alas, I'm without easy access to one at the moment), but the supporting information is freely available -- including a movie of a single nanodiamond inside a HeLa cell.