Solid Materials refers to materials which appear to be carved out of a solid block, rather than just applied to faces of a sculpted object.
SketchUp places materials on faces, and does allow for some mapping which wraps onto surfaces. However, if you want a realistic rendering from an object sculpted from wood, you will need to use a Photorealistic Ray Trace program, such as IRender nXt to create the rendering.
Here is a sculpted SketchUp model with a solid wood grain mateiral applied:
Here is a closeup of some of the solid material detail: