A metal is considered "precious" if it is rare and is of high economic value. Under these two factors, nine metallic chemical elements qualify as precious metals. These are, in no particular order, gold, palladium, silver, rhodium, iridium, ruthenium, platinum, osmium, and rhenium.
So that while the chemical element tellurium is considered one of the rarest element in the Earth's crust (its mass abundance being the same as that of rhodium, iridium, and ruthenium), its estimated price is only less than 5 U.S. dollars per troy ounce, and, therefore, cannot be considered a precious metal. The same can be said of bismuth, which has a mass abundance half less than that of palladium, but with a price of only a little more than a tenth of tellurium's price.
In terms of mass abundance, expressed in parts per billion (ppb), here's how the nine precious metals are ranked (rare to rarest): 9th: silver (75 ppb); 8th: palladium (15 ppb); 7th: gold (4 ppb); 6th: osmium (1.5 ppb); 5th to 3rd: iridium, rhodium, and ruthenium (1 ppb); 2nd: rhenium (0.7 ppb); and 1st: platinum (0.003 ppb).
The ranking appears different when the estimated world market prices, per troy ounce, of these nine precious metals are considered (estimates as of January 2010):
1st: Rhodium - USD2,750
2nd: Platinum - USD1,555
3rd: Gold - USD1,131
4th: Palladium - USD424
5th: Iridium - USD408
6th: Osmium - USD380
7th: Rhenium - USD194
8th: Ruthenium - USD173
9th: Silver - USD18
Of the nine precious metals, gold and silver are the best known. Apart from being traditional coinage metals (along with copper), both gold and silver are also well known for their uses in jewelry and art; they likewise have certain industrial uses.
A third important factor that will qualify an element as a precious metal, as may be noted from the uses of gold and silver above, is that it should not be radioactive. Thus, the chemical elements actinium, polonium, and radium are not considered precious metals because they are highly radioactive.
Compared to most other elements, precious metals are chemically less reactive, have higher melting points, and are more ductile - properties that make them ideal for many commercial and industrial applications.
In earlier times, precious metals were mainly used as currency. Today, however, they are highly regarded as investment and industrial commodities. Four of them, in fact - silver, gold, platinum, and palladium -, are minted into coins or cast into ingots and traded on commodity markets. All four are assigned the ISO 4217 currency code.
Hence, the role of precious metals as investments, on top of their practical use, drives the demand for them. For thousands of years, precious metals have demanded prices that are much higher than those of common industrial metals, such as nickel (about 36 cents per troy ounce) and copper (about 11 cents per troy ounce). As a matter of fact, there have been significant increases in the prices of precious metals at the turn of the Twenty-First century. This has encouraged many enterprises to go into precious metal recycling.
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