Replicating nature can be difficult. Natural materials are usually the most difficult to recreate in a satisfying way using computers, mainly because the chaos of nature is not the best fit for the orderly logic of an electronic machine.
Too often, we even see cubes that look obviously computer-generated because of the neatness and regularity of their shapes or surfaces. Actually, in reproducing true to life a natural object (or material as well), we have to start from the absolute regularity of the computer simulation, and then blemish it step by step in a controlled way to reach a more natural look.