Aluminium is a common element. Refining it to a metal is a very difficult process (compared to other metals). It was so valuable that it was part of the showing off a young United States did for the Washington Monument. http://www.tms.org/pubs/journals/jom/9511/binczewski-9511.ht...
> Frishmuth proposed that the pyramid be made of aluminum at a quoted price of $75, and if he was unsuccessful in casting it in aluminum he would cast it in aluminum bronze and plate it with gold for a total cost of $50, or, if platinum-plated, for $75. Aluminum bronze was an alloy of 90 parts copper and 10 parts aluminum and was quite abundant on the commercial market in 1884. The principal manufacturer was the Cowles Brothers of Niagara Falls, New York, who used a patented thermal process employing an electric resistance furnace and a raw material containing aluminum oxide and copper. The aluminum produced was always combined with the copper and could not be separated from it.
> Roman historian Pliny the Elder recorded a story about a metal that was bright as silver but much lighter, which was presented to the Emperor Tiberius (reigned 14–37 CE), who had the discoverer killed in order to ensure the metal would not diminish the value of his gold and silver assets. Some sources suggest a possibility that this metal was aluminium; this claim has been disputed.
Made with electrolysis of a molten aluminum salt. Requires a lot of electricity. Back in the 1850s they made it with expensive chemical reactions and the french elite used to wear it as jewellery.
Aluminum forms very strong chemical bonds, so it's difficult to reduce it to its base form. Aluminum oxide is one of the most chemically stable materials there is. In WWII, being able to effectively refine aluminum (using lots of hydroelectric power in the Pacific Northwest) was one of the key economic factors that lead to an Allied victory.
For the entire commercial history of aluminum, it's been produced through molten-salt electrolysis (the Hall-Heroult process).
