Aluminum
Alum, as aluminum was called in the Middle Ages, was rare, but highly valued. The Ottoman Empire placed huge export taxes on its sale, and when large stores were discovered in Italy, they were described as “the death of the Turk,” and the Pope himself forbade its importation from the Ottomans in order to start a trade war. Dozens of prominent chemists over the years attempted to synthesize it and failed, until 300 years after it was established as a metal, Friedrich Wöhler managed to isolate it in a relatively pure form, albeit at double the cost per ounce of gold. Napoleon craved it for weaponry, and gave his most distinguished guests aluminum utensils instead of gold ones.
In a Roman era tale, the Emperor threw a metal cup to floor because it wasn't gold. Seeing the cup didn't break and could easily be returned to form with a hammer, the Emperor was eager to find the maker. The smith, who made the cup, was pleased to report that he alone knew how to produce the material. The Emperor ordered his execution so as not to diminish the price of gold.
139 years ago today, an American inventor used electrolysis to extract aluminum from aluminum oxide, a process that eventually resulted in reducing the price of aluminum by a factor of 200, making it affordable for many practical uses from soda cans to the Wright Flyer.
Today, more aluminum is produced than all other non-ferrous metals combined. The process was discovered by Charles Martin Hall.