Personally, I'd use one on my Sapphire Cub if I ever get one. There's a bunch that purchased that only come as Uncommon like the Amen Vale Lashling and Crimson Lashling.
When I get stones for things I don't care a whole lot about, like mechanical. My daughter and I come up with a game. We'll each pick 2 or 3 or 4 that I have that's not rare and then roll a d4 or d6.