On the end of the block you see these concentric circles. Along its length you see lines that are the veins. However the pattern exists throughout the entire block. If you cut or carve the wood it reveals the pattern inside. Similarly an onion consists of concentric spheres that are visible only when you slice it.
Marble stone consists of wavy layers of colored sediments that harden into rock.
These solid patterns can be simulated using mathematical functions. Other random patterns such as granite or bumps and dents can be generated using a random number system and a noise function. In each case, the x, y, z coordinate of a point on a surface is used to compute some mathematical function that returns a float value. When used with color maps or pigment maps, that value looks up the color of the pigment to be used. In normal statements the pattern function result modifies or perturbs the surface normal vector to give a bumpy appearance.