Natural, minimally processed materials over synthetic re-creations. 

Materials that reflect time and use. 

If man-made materials are used, the closer the representation is to the real material the better, as some of those inherent properties may be suggested. 

Consider especially the goal Information Richness & Complexity.  Humans are able to perceive very small scale in the structure of materials.  This detail should be available at both the smallest scales, but also at an intermediate and large scale (5).