Old stones can be cleaned by putting them in the dishwasher, Let the stone cool to room temp before you use it. If the stone surface needs some love then you can resurface them with a diamond stone.
It's fairly blurry so hard to tell, but it looks like the surface is pretty gouged up. It can probably just be flattened and resurfaced on some wet/dry sandpaper on top of a piece of glass or countertop.