Creating Man-made Materials in Cycles - Blender Cycles Materials and Textures - page 177

The bump pattern, based in part on the output of the veining
24. Save the file as Wood.blend .
How it works...
From steps 1 to 4, we built the basic shader using the usual Diffuse BSDF and Glossy BSDF
nodes, mixed by a Fresnel value and multiplied by the values of a medium gray color.
From steps 5 to 18, we built the color of the wood's veins, adding three procedurals to
be used as splitting factors for the two wood colors set in the penultimate MixRGB node.
Using the last Multiply3 node, we made the color more saturated (actually, we multiplied
the values by themselves).
From steps 19 to 23, we built the bump using a noise grain summed to the veins' values
by the Add2-Math node. We set a low value for the bump's Strength value, but you can
use higher values (together with higher roughness values) to obtain less polished surfaces,
which can give you different kinds of wood in the output.