Various species of lichens are common on rocks throughout the world.

These tiny composite organisms are remarkable in that many species can slowly break down their substrate (rocks) by chemically and even physically means. This action is accomplished by several means beginning with the penetration of the fungal filaments (hyphae) into the rock, swelling and shrinking actions of body (thallus) of the lichen, the excretion of organic acids, especially oxalic acid, which can dissolve minerals and other actions. Numerous studies show that the weathering of rocks and minerals is accelerated by the presence by lichens. Of course, the speed of decomposition by lichen activity is measured in geologic time rather than a human lifetime. It may take several hundred years, or as many as tens of thousands of years to degrade softer rocks. It may take hundreds of thousands of years or longer to degrade harder stones. The rate of weathering depends upon the different species of lichens and the mineral composition of the rocks. See the excellent paper Weathering of rocks induced by lichen colonization—a review by Jie Chen, Hans-Peter Blume and Lothar Beyer for a more detailed account of these actions.  

We will look at one example of lichens on rocks using Murphys stones from central California as an example. These stones, described by collectors as a combination of limestone and quartz, are found near the small community of Murphys, California in the foothills of the Sierra Nevada mountains in Calaveras County. It was once a gold mining location. Murphys stones are collected and exhibited as good examples of scenic landscape viewing stones. Most of these stones suggest landscapes as the one illustrated below. 

This stone is 40 cm wide, 22 cm high, and 22 cm deep without the base. Jerry Braswell made the base, and the photograph was taken by Sam Edge.

Figure-shaped stones are rare among this type of stone. One of the best examples of this is a bird-like shaped Murphys stone collected by Ken McLeod. These stones have lichens in the crevices and occasional small black lichens on the surface. Several types of lichens can be found on a single stone. Many of the lichens are of the crustose type that grows tightly pressed on the rock. Another type found on these stones has a form that is tightly grouped and appears as tiny pebbles (squamulose). In deeper crevices, small branched lichens occur that have leaf-like structures (foliose) or free-standing branch-like structures reminiscent of types of moss. All of the types can be found on Murphys stones.

This Murphys figure stone is 26.5 cm wide, 18.5 high without the base. The base was made by Jerry Braswell, and the photos by Tom Elias. The close-up is a view of lichens on the surface of the stone. 

It is common for Murphys stones to be partially covered with lichens. According to our friend and professional stone collector and dealer, Ken McLeod, the stones occurring in exposed areas with abundant sunlight have more lichens than rocks occurring in shaded locations. McLeod likes the lichens on these stones, but he removes them to make them more sellable to customers. He does this by soaking the lichen-covered rocks in hot water and bleach (sodium hypoclorite) overnight in an insulated cooler. He then brushes away the lichens of each stone to have a cleaner-looking Murphys stone. Both the stones illustrated above appear to have been "cleaned." However, small or sparse patches of lichen remain on both stones, and signs of new lichen growth are evident. The presence of lichens in the crevices of these stones helps to add a perception of depth to the object while adding subtle color.

The cleaning process does not alter the shape or form of the stones. However, the chemical and mechanical cleaning methods can alter the stone's surface. Bleach is a strong oxidizing agent that breaks chemical bonds between parts of molecules apart. This chemical is used to remove the color from fabrics and to remove stains. This process does clean most of the biological material off the surface of a stone, but not in an overly conspicuous way. Thus, cleaning stones in this method is acceptable since it allows us to see the features of each stone.

The section on cleaning stones in the Smithsonian's book Rock and Gem, The Definitive Guide to Rocks, Minerals, Gem and Fossils (2008) states that even the heat from hot water can cause certain minerals to crack. This excellent reference recommends, as a general rule, to clean specimens as little as possible, starting with the gentlest methods. This book also recommends avoiding soaps as they contain additives that can penetrate rocks. Viewing stone collectors are presented with a dilemma—to clean or not to clean. 

Lichens have a more pronounced effect on calcareous or limestone rocks than other rock types. All species of lichen occurring on limestone produce oxalic acid, which effectively dissolves limestone and dolomite. Since Murphys stones are primarily limestone, we can attribute at least a portion of their form to degradation by lichens which are not just resting on the surface of these limestone rocks but have become a fused part of the surface and near-surface layers. Lichens cannot be removed without altering the surface. Thus, we can consider lichens to be a naturally occurring part of these stones and be receptive to displaying stones with their lichens. To alter the stone with chemical and mechanical treatments is to render them as enhanced stones. The truly natural as-found Murphys stone usually includes one or more species of lichens. Their intimate association with rocks is different from just the accumulation of dirt or silt on the surface of a stone. At the same time, we should not criticize those who clean stones with the intent of exposing the stone to appreciate it or use it for meditative purposes. 

Now, let’s look at a natural occurring Murphys stone as it was found. This stone has approximately 60% of its surface covered with different species of lichens. This stone may be suitable for a seasonal autumn display. The colors suggest a fall scene with colors that match autumn foliage in cool to cold temperature regions of the world. Traditionalists may argue that lichens are masking the features of the stone and that we should only focus on the features of the stone itself even if it means moderate to aggressive cleaning of a stone. To clean this stone and remove the lichens is to take away from the some of the naturalness of this stone.