Advice on cleaning a rock.

[Jamastiene]

I lost my old petrified rock when we moved. I used it as a nice looking coaster that fit my home's decor. I recently found another rock that is flat enough to work as a new coaster and look nice.

I have one problem though. Other than rinsing it off, I am not sure how to keep it from leaving gritty bits of debris on the table. I know I am going to need to do more than rinse it off and glue some felt on the bottom.

Does anyone know of any tricks to laminate/lacquer/seal a rock to keep the little crumbly bits from ruining my tabletop? Even advice on how to clean it better than just rinsing it would help.

Thanks.