Noble metals are metals that resist oxidation and corrosion in moist air. The usual list is gold, silver and the six platinum-group metals: platinum, palladium, rhodium, iridium, osmium and ruthenium. Their stability explains why they dominate fine jewelry and coinage, and why gold pieces thousands of years old still shine.
What makes a metal noble
Most metals react with oxygen and water. Iron rusts, copper turns green and aluminum forms a thin gray oxide skin. Noble metals react weakly or not at all under ordinary conditions. Gold is the extreme case. It does not tarnish in air, and single common acids leave it alone. It does dissolve in aqua regia, a mixture of nitric and hydrochloric acids, and in the cyanide solutions used to extract it from ore.
The word has more than one definition. The chemistry and jewelry sense is the one above, based on corrosion resistance. Physicists sometimes use a stricter definition based on electron structure that covers only copper, silver and gold. Some lists also add mercury or rhenium. For jewelry, the group means gold, silver and the platinum-group metals.
"Noble" and "precious" overlap, but they describe different things. Precious refers to rarity and market value. Gold, silver, platinum and palladium are both noble and precious. Osmium is noble and rare, yet it has no place in jewelry.
Silver is the weakest member of the group. It does not oxidize in clean air, but it reacts with sulfur compounds to form black silver sulfide, the tarnish that collects on sterling silver. Some chemists leave silver off the noble list for this reason.
Gold
Gold is the most corrosion-resistant metal in everyday use and the most malleable of all metals. It can be beaten into leaf thin enough to let greenish light pass through. It melts at about 1,064 °C and has a density of about 19.3 g/cm³, so a gold ring feels heavy for its size.
Pure gold is soft, so jewelry gold is almost always an alloy. The karat system describes how much gold the alloy contains, and the other metals in the mix set its color: yellow gold, white gold or rose gold.
Gold occurs in nature as native metal, in quartz veins and as grains in river gravels. It almost always carries some silver. When the silver content passes roughly 20 percent, the natural alloy is called electrum. People have worked gold for a very long time. Grave goods from the Varna cemetery in Bulgaria, dated to the fifth millennium BCE, are among the oldest worked gold objects known, and the goldsmiths of ancient Egypt left some of the best-preserved examples.
Silver
Silver conducts heat and electricity better than any other element, and polished silver reflects more visible light than any other metal. It melts at about 962 °C and has a density of about 10.5 g/cm³. Like gold, it is too soft to wear pure for long, so most silver jewelry is sterling, an alloy of 92.5 percent silver and 7.5 percent other metal, usually copper.
The platinum-group metals
Six metals sit next to each other in the periodic table and occur together in the same ores. Chemists split them into a light group (ruthenium, rhodium, palladium) and a heavy group (osmium, iridium, platinum). All six are hard to melt, and all resist corrosion.
| Metal | Symbol | Density (g/cm³) | Melting point (°C) | Jewelry use |
|---|---|---|---|---|
| Platinum | Pt | 21.45 | 1,768 | Solid jewelry, usually 950 |
| Palladium | Pd | 12.0 | 1,555 | Solid jewelry and white gold alloys |
| Rhodium | Rh | 12.4 | 1,964 | Plating |
| Iridium | Ir | 22.6 | 2,446 | Hardener in platinum alloys |
| Osmium | Os | 22.6 | about 3,030 | Not used |
| Ruthenium | Ru | 12.4 | 2,334 | Hardener in platinum alloys, dark plating |
Platinum was worked by pre-Columbian goldsmiths on the coast of what is now Ecuador and Colombia, long before Europeans studied it. The other five were found in the residues left after dissolving platinum ore. The English chemist William Hyde Wollaston identified palladium in 1802 and rhodium in 1804. He named palladium after the asteroid Pallas, discovered the same year, and rhodium after the Greek word for rose, for the color of its salts. Smithson Tennant found iridium and osmium in 1803. Iridium takes its name from Iris, the rainbow, because its salts come in many colors, and osmium from the Greek word for smell, for the sharp odor of its oxide. Ruthenium came last, isolated by Karl Ernst Claus in Kazan in 1844 and named after Ruthenia, a Latin name for Russia.
Osmium and iridium are the densest natural elements, at about 22.6 g/cm³. Osmium is brittle, and its oxide is volatile and toxic, which keeps it out of jewelry. Iridium and ruthenium appear in small amounts as hardeners in platinum. Rhodium, very bright and very hard, is used as a thin plating on white gold and silver.
Why noble metals suit jewelry
Here is why these metals have held their place for thousands of years.
- They keep their color and luster. A gold ring does not rust or corrode, and platinum and palladium stay white without plating.
- They are workable. Gold, silver and platinum can be hammered, drawn into wire, soldered and cast, which lets a goldsmith build fine settings for stones.
- They rarely irritate skin in their pure or high-purity forms. Most metal allergies in fine jewelry come from alloy metals such as nickel.
- They can be melted and reused without loss, so old jewelry is routinely refined and made into new pieces.
The jewelry metals overview compares these metals with alternatives such as titanium and steel, and the hallmarks guide explains the purity stamps used for each.
Frequently asked questions
Is silver a noble metal?
It depends on the definition. Silver resists oxygen and water, so most jewelry and chemistry references include it, but it tarnishes in the presence of sulfur, and some chemists leave it off the list.
Is copper a noble metal?
Not in the usual chemistry sense, because copper oxidizes and corrodes in air. Some physics texts call copper, silver and gold noble metals because of their similar electron structure.
Why is osmium never used in jewelry?
Osmium is brittle and very hard to work, and when heated in air it forms osmium tetroxide, a toxic, volatile compound. Iridium and ruthenium, its neighbors, are used as hardeners instead.
What is the difference between noble and precious metals?
Noble describes chemical behavior, mainly resistance to corrosion. Precious describes rarity and value. Gold, silver, platinum and palladium fit both terms.