A metal is deemed to be precious if it is rare. The discovery of new sources of ore or improvements in mining or refining processes may cause the value of a precious metal to diminish.
An interesting case of a once-precious metal that is now common is that of aluminium. Although aluminium is one of the most commonly occurring elements on Earth, it was initially found to be exceedingly difficult to extract from its various ores. This made aluminium more valuable than gold. Bars of aluminium were exhibited alongside the French crown jewels at the Exposition Universelle of 1855, and Napoleon III was said to have reserved a set of aluminum dinner plates for his most honored guests. Over time, however, the price of the metal gradually dropped; the discovery of the Hall-Héroult process in 1886 caused the high price of aluminium to permanently collapse.
The rarity of other precious metals may shift in the future; the supply deficit of silver - 300 million troy ounces (9 million kg) more silver are currently consumed per annum than are mined - may ultimately cause it to become rarer than gold.
Wikipedia® is a registered trademark
Fine precious metal jewelry
Diamonds, gold, silver, platinum, and semi-precious gemstones...