Hey Joey,
I am going to have to agree with Hal here. I think that what you do is setup the lesson in this manner:
1. Play a video for them, or record two players in game digging up something and they accidentally break the fossil! OH MY!
2. Discuss with them the ramifications of doing such a terrible thing and how it could mean the loss of a new species forever! What if it was the only one of its kind!??!?!?
3. Being careful is important with this type of work, its not about the speed, but about the accuracy...which is why we don't use TNT to blow up the ground looking for fossils! Instead...we use delicate tools that some might seem down right slow (only give them wooden tools, they are slow and take longer to dig)
4. Give them some smaller sample site...to try it out. Let them see a fossil outside of the ground..notice the distinct orange (whatever color block you choose) color of the fossil...that students is what we are looking for!
5. Let em go...and I bet the lesson goes 100% smoother!
I am more about teaching them how to do things properly...than how to prevent them from making mistakes. Learning from failing can be very important.
Let me know how it goes.
MisterA