what type of stone?...moss rock, slate, brick?...I have used some citrus based strippers with ok results on some flagstone before...it is a messy process but it did work in the end.
A real good method of doing this is Soda Blasting. This is not a do it yourself type of project as it requires some special expensive equipment. But done correctly you will end up with no damage to the stone or cement and a finish as though the fireplace was just installed.
Soda blasting is much like sand blasting only Baking soda in a larger sand like form is used. It is blasted out of the hose at high pressure. When it strikes the surface it explodes removing a slight layer of finish in the process. The result is that it does not eat away the surface below. Once done, it then simply gets vacuumed up and flushed with water to dissolve the soda dust and all that is left it the finish to put into the garbage.
If it's inside, I would just paint it back like stone.
If it is gloss, you will have to rough it up. You can sand it a little with a combination of sandpaper and steel wool and then put a coat of flat varnish on it. I guess they did this to seal the stone so it could be kept clean. I have no idea what to use to get the varnish off and if you find something it won't damage the look of the stone...???? Never heard of such. You will hav to find an expert.
Great idea Jeanette, you sound like the expert now.
There is a product called 'Dead flat varnish'.