I took this exercise from Concept Cookie, it's a bit old, but i promised my self to play with it as soon as i can, so here it is!
Studying things it's allways usefull so i spent the day trying to understand some materials and their reaction to light!
The most hard for me was metal and i think i should work a little bit more on gems, because this one looks more like ice maybe (is it the color?), and wood cortex too. If you want to test yourself you can find a psd file one the Concept Cookie page (link above) and you can also see the other's results.
Have fun, i had it!