At one time, aluminum was considered a precious metal, much like silver and gold. This was not due to scarcity, since aluminum is 8% of the Earth, making it the most abundant metal, and the third most abundant element. However, the process to separate aluminum metal from its ores and minerals (e.g. bauxite, feldspar) was so difficult that it cost as much as silver.
The capstone of the Washington Monument is made of aluminum, which was at the time a precious metal. P. H. McLaughlin, the master mechanic of the project, is seen above setting it in place.
The Hall-Heroult process, which is the industrial process for producing aluminum metal, was invented in 1886, soon after the Washington Monument was completed. Today we consider aluminum a relatively inexpensive material. A similar discovery will not cause the price of the precious metals to fall, since they are scarce in nature, unlike aluminum.