Posted: 31/08/2012 at 12:19
It may seem odd that coal made from trees and other organic matter can contain heavy metals such as lead, mercury and (radioactive) uranium and other pollutants but it does. The contaminants come from the normal geological erosion of rocks and are washed into the swamps where coal is formed. These are mainly concentrated in the boundary seams of coal but you have no way of telling where in the seam the coal came from. The heavy metals can leach out of the coal to pollute water courses. This largely a problem for power stations where they store millions of tons of the stuff.
If the layer isn't too thick I would dig it out first especially if planting fruit and veg.
The pinky bits might be brick debris from the previous demolition of the coal shed.