h2g2 The Hitchhiker's Guide to the Galaxy: Earth Edition

This is, of course, an excellent and interesting Entry. However, although I know that Gnomon was trying to avoid too much chemistry, I was disapponted that he didn't mention these two aspects.

Fool's gold, or iron pyrites is a sulphide of iron which is gold in colour and frequently occurs with gold and copper minerals. Hence, this was frequently a disappointment to prospectors who thought they'd found a seam of gold.

http://parks.afcd.gov.hk/newparks/eng/education/classroom/1/mscnt13.htm

Phyto-mining: When there are low-grade ore deposits it has been found commercially viable to extract gold deposits using this method (a paper in 'Nature in 1997 or '98) This involves growing crops such as chicory, which naturally accumulate gold in their leaves To be economically viable a plant would need to accumulate around 17 micrograms of gold. Scientists succeeded in harvesting one plant, Brassica juncea, with 57 micrograms. More typical values were between 9 and 19 micrograms.
In order to mobilise the gold and facilitate absorption by the plant, the soil has to be sprayed withammonium thiocyanate.

P.S Brassica juncea is leaf mustard.