Geology is a passive skill, similar to the various lore skills. Generally you will not be aware that you are making use of the geology skill. However, it can be of use during mining, stone or metal working, and some other situations, such as when you examine cave walls in the underworld. A higher level of skill in geology may enable you to identify the types of rocks, metal, or ore that you come across in your travels through the world.
See also: lore