My son has an interest and is bored this evening so I did a little looking around and found:

A gemstone coloring page at Crayola.com and a wealth of resources at the Mining Information Institute website.  I am planning to use these copywork pages over the next few weeks as well. 

I also used this resource from abcteach.  If you need more materials, just do a search using "Earth layers worksheet" and "rocks and minerals worksheet."

Happy rock hunting!