The resistive properties of coal are no surprise (7 orders of magnitude) since resistors have been made from carbon film and carbon composite for 50 years. I think the big problem there is whether the coal has a consistent resistence.<p>They talk about how much work it is to purify silicon, but won't the same problem exist for coal? Any semiconductor has to be doped to form transistors, diodes, etc. It also has to be pure so that the tiny geometries used for these devices can be reliably transferred from the masks.<p>Call me skeptical, but the material to make purified silicon is <i>more</i> abundant than coal (and cheaper to mine). I think this will ultimately prove cost prohibitive.