It's small: one room filled with some interesting specimens.
It's not the Smithsonian, but it only costs about a buck to look around and enjoy. In addition to mineral samples there are also fossils, and English language written explanations that need to be returned.
If you find the place and the door is locked, don't dispair. Ring the doorbell and you will be buzzed in where there is a reception desk where you can pay the three zloty entrance fee.
Good for a 15-20 minute visit.