You fireplace rock is indeed covered with lichens-not moss.
Please DO NOT attempt to make them grow and DO NOT use any of the above formulas. You will end up killing your beautiful lichens and ruining your fireplace walls.

This info comes from: "A Rocky Mountain Lichen Primer" by Corbridge & Weber, University Press of Colorado, 1998. Dr. Corbridge is a Law professor at University of Colorado and an amature lichenologist. Dr William "Bill" Weber is Professor Emeritus(Botany), was curator of University of Colorado's Herbarium, and author of numerous "Colorado Flora" books, among others.

"Gardeners and homeowners should be informed that these lichens can be killed by care. They do no require any watering, they grow very slowly (few people will be able to live long enough to see any growth), and they are sensitive to pollution of many kinds. In the home, lichens on the fireplace, if watered, may be quickly destroyed by the molds that are prevalent in the air of the rooms. When they accumulate dirt, they are difficult to clean. Some crusts may be cleaned by wadding up white bread and using this as a "wallpaper cleaner". Foliose and fruticose lichens are extremely fragile, perhaps a soft airbrushing might help. Do not attempt to make them grow, despite what you may be told by ignorant dealers.....If you have lichens, don't water them."
He goes on to say that a New York Times garden editor once suggested that a spreading a coat of peanut butter over the lichens would help them turn green. It produced nothing but a thick coating of green mold. He has seen expensive rock walls in malls destroyed from scrubbing them with a wet mop. He maintains that "Even normal watering in the garden will eventually kill them".

Hope this helps you.

if you still follow this thread here are some versions of the recipes you have been trying to find. I personally have been meaning to give them a try but you know how that goes...,

I can't remember where I got these.. so my apologies to the author(s)

"How to Grow Moss for Your Garden"
Author Unknown

Adding moss to a garden is a beautiful way to enhance its appearance, and with this easy and inexpensive recipe, you'll enjoy growing moss as well.

Steps:
1. Put a handful of the moss you want to grow into a blender.
2. Add 1/2 tsp. sugar and one can of beer (the cheapest brand). You can substitute buttermilk for beer if you want.
3. Blend just long enough to mix the ingredients and break down the moss.
4. Spread the soupy mixture with a spatula over the ground or rocks where you want the moss to grow.

Tips:
Remove as much dirt from the moss as you can before you start.

Moss grows best in shady areas.

HOW TO GROW YOUR OWN MOSS !
This will require a shaded & fairly moist spot that is not going to get washed by rain or sprinklers...
In a blender, combine 1 quart of buttermilk, 1 Tbsp. corn syrup, 1 cup beer of choice & 2 cups finely chopped , freshly harvested moss w/ developed spores or use 2 to 3 packages of Moss Spores A hand-full of brown fallen evergreen needles will help but is not required. Blend the mixture until it's smooth; adding more beer if you desire a thinner mix or more fresh moss for a thicker paste. Simply paint or pour the slurry onto the surface of whatever you want mossed and keep it moist. Do not use this mixture on bonsai... You will have to grow it separately on a page of newsprint dusted with a thin layer of peat and transplant it onto your bonsai when it is fully grown.
Keep in mind, many mosses are not fast growers and may need 1 or 2 seasons to impress you depending on moisture levels, shade requirements and soil composition. Your ideal soil Ph should be near 5.5 ( too sour for most grasses ). Use sulfur, aluminum sulfate or ferrous sulfate to sour the soil in a shade garden. We've experienced good results using various quantities of the ingredients in this recipe and by being religious about the growing conditions that most mosses enjoy...moist shade.

HTH's
Butch

Moss rock fireplaces do not have moss on them. If they did at some point have moss on them, there is absolutely no way you could keep it alive anywhere indoors let alone on stones around a fireplace. Do not paint anything on rocks around your fireplace.

As Rosa pointed out, but everyone ignored, these moss rocks are named for their lichen growths which provide coloured patches on the rocks. I don't honestly know if these remain alive indoors on the fire surround but they are probably waiting in limbo for suitable conditions to grow again. If they are alive then you could splash a little water on them, but don't keep them wet for too long, and they might freshen up. If they are dead then you are stuck.

Anyone who wishes to grow moss on anything should ideally move to Britain or possibly the Pacific side of the Cascades. Failing that, check out The Moss Forum.