They are probably mounted on backer board. Chip out the mortar from between the stone with a hammer and concrete chisel then break out the stones. You will need to replace the backer board afterwards.
Paint it white! :)
Dont paint it, chip it off. You are right. It is quit awful.
Courtney is right you could paint or whitewash it. That would be the cheapest and easiest solution. Adding a new mantel would help too. Maybe a nice beefy looking piece of rustic looking wood.
Skim coating is one option but I think the stone is too thick to make it look good. I would follow the advice of Zard and chip off the stones and replace the backer board. It will give you a cleaner more modern look to the fireplace.