Excellent thinking! The paintbrush is a good choice!

You can use the paintbrush to gently push away
the fur and dirt from whatever objects, if any,
you find inside your "rock". That way you won't hurt
the items, but you can still get them clean
enough to get a good, close-up look at them.

Also, your hands have a natural oil on them which gets transferred to whatever you touch. Using the paintbrush instead of rubbing the objects clean with your fingers may help preserve them if they turn out to be valuable. It is slow work, but it pays off.

Archaeologists often use small brushes like this when they are digging for old artifacts or lost civilizations. Museum curators sometimes use soft brushes to clean the dust off of their collections.

Do you use paintbrushes
around your house for any
reason other than painting?

Try another of the tools you collected.

Or, if you are done taking the object apart,
you can continue with your investigation...