I love the idea of minerals & rocks as middle names (but I am a geologist!).
Garnet, for instance is a mineral created under the highest pressures & high temperatures (and it's very beautiful). To me a name like that would impart some inspiring character traits... I like to think anyway.
Kyanite is the same as Garnet, but is a long, thin shape (as opposed to a red soccer-ball), and a brilliant deep blue.
Olivine is also a fantastic one. A brilliant green, and can be formed in lava or magma (lava that's not on the surface)

If minerals tickle your fancy, this website has stacks of info (maybe too much...), but it also has mineral names in different languages (incase the '-ite' on the end of almost everything isn't to your taste). Kyanite: Kyanite mineral information and data.