Stone contains minerals and deposits that are organic, which supplies
nutrients to the lichen spores.
Lichen is the most difficult affection to prevent and when the spores have gotten a hold on the stone surface, it is hard to remove. Lichen survives strong cold and extreme heat for long time, due to its high survivability.
The affection's and the growth's development stops only when all spores, rests from old affections and all of the stones pores have been thoroughly cleaned. Sterilization and disinfection is needed.