The capstone of the Washington Monument is a 9-inch tall aluminum pyramid, placed on December 6, 1884, to finish the 555-foot obelisk. Aluminum was chosen because it was as valuable as silver at the time due to the difficulty in refining. At the time it was the largest piece of cast aluminum ever made.
Two years later, newly discovered industrial processes brought the price of aluminum down to earth but the original capstone is still there.
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