Nokta FoRs Gold, a Gold Cube, 2 Keene Sluices and Lord only knows how many pans....not to mention a load of other gear my wife still doesn't know about!
The pitting is likely an impression of the rock in which this piece was once a layer. The bond between the rock and this layer would have broken, or the rock would have been eroded, leaving this piece of the layer with the impressions.