Aluminium was once more valuable than gold, and the tip of the Washington Monument is an aluminium pyramid.
Why it is true
Although it is the most abundant metal in Earth's crust, it was extremely hard to extract from ores. In 1884 the small 2.8 kg pyramid crowning the monument was a luxury item. Two years later the Hall-Héroult electrolysis process made aluminium cheap.
Source: Science History Institute
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