A stone or rock covered entirely or partly in moss gives the impression of great age… and in fact, if you leave a rock in a suitable location, moss will grow on it… eventually. Fortunately, you can speed things up if you transplant the moss yourself.
Try this method. Using your handy food processor (I recommend not telling your spouse!), blend together a handful of green moss gathered from your property, a handful of pottery clay* (available in handicraft stores) and about 2 cups (500 ml) of water. Apply the mixture to the rock with a brush.
Keep the rock moist for the next 5 weeks by misting 2 times per day or, better yet, by installing a temporary misting system. By then the moss will be well established and you can decrease your misting. After the first summer, the moss will be able to cope with local conditions without further intervention.
Note that the idea that the moss only grows in the shade and under high humidity conditions is false. On the contrary, there are mosses suitable for all conceivable conditions: dry or wet spots, shade or full sun, acid or alkaline surfaces, etc. Ideally, therefore, you should harvest mosses growing under about the same conditions as those that exist on the rock where you want them to grow.
* Other recipes advocate the use of buttermilk, yogurt or beer instead of clay, but the latter gives much better results because it helps moss particles to stick to the rock surface and, unlike buttermilk and yogurt, doesn’t result in fungus growth.