The latest candidate is rust: iron oxide. The November 29 issue of Science has a very beautiful cover image of a scanning electron micrograph of iron oxide crystals. These nanoflowers are not merely pretty. They can replace expensive, rare ruthenium, rhodium and platinum catalysts in key organic chemistry reactions: for example, the production of anilines. Perhaps it is a good time to look again at all over-familiar materials.
Science, 2013, Vol 342, pp. 1054-55.