Everyone’s heard of platinum, but far fewer people know that it’s just one of six precious transition metals classed as platinum group metals (PGMs). The other five platinum group elements are palladium, rhodium, ruthenium, iridium, and osmium.
The six platinum group metals share similar physical appearances and chemical properties. They are all dense, noble metals with a lustrous silvery color.
According to the United States Geological Survey, other distinctive properties of platinum metals include:
- Extremely high melting points
- Highly resistant to tarnish, corrosion, and most forms of chemical attack
- Typically found together in the same mineral deposits, or as by-products of copper and nickel production (mainly from sources in South Africa, Zimbabwe, and Russia)
- Useful in the manufacture of electrical components, catalytic converters, fuel cells, and nitric acid, and in various other industrial applications
Read on to find out more about the history and characteristics of the individual platinum group metals, and how each is used within the jewelry industry.
Platinum is the eponymous and most abundant member of the platinum group. Prized for its silvery-white sheen, rarity, and resistance to tarnish and corrosion, it has been used in the making of platinum jewelry for thousands of years.
Much of the value of platinum derives from its extraordinary scarcity. It’s 30 times rarer than gold, and as such, has often been priced higher. However, the prices of precious metals are always fluctuating, and at the time of last updating this guide, gold is the more expensive of the two.
Interestingly, platinum wasn’t always recognized as valuable. When it was first encountered by 18th century Spanish miners in South America, they regarded platinum as an unwanted impurity in silver ore, derisively calling it platino (meaning “little silver”).
Platinum is the third-densest natural element known to exist, placing just behind its fellow platinum group metals osmium and iridium. This makes platinum jewelry unusually and luxuriantly heavy – you will definitely notice a difference in weight compared to gold or silver jewelry.
Platinum is malleable and ductile, which means it can be crafted into different shapes without cracking or losing strength. These are desirable qualities for a jewelry metal, and help to explain platinum’s popularity.
However, because it’s a relatively soft metal, pure platinum is rarely used in jewelry making. Instead, most platinum jewelry is made from an alloy (mix) of platinum and other metals. Alloying platinum with small amounts of harder metals increases its hardness and durability.
In the United States, platinum jewelry is usually made from platinum-rich alloys of very high purity. Popular alloys include:
- 95% platinum and 5% ruthenium
- 95% platinum and 5% cobalt
- 95% platinum and 5% iridium
- 90% platinum and 10% iridium
By comparison, karat gold alloys are typically 75% pure (18k gold) or 58.3% pure (14k gold).
Platinum Quick Facts Chart
|Chemical Symbol||Pt||Mohs Hardness||4 to 4.5|
|Density||12.39 oz per cubic inch||Year Discovered||1735|
|Melting Point||3,215 °F (1768 °C)||Price per Oz||$980 USD (as of Aug 2023)|
Apart from platinum, palladium is the only one of the platinum group metals used in large amounts within jewelry alloys. The standard formulation is 95% palladium, with 5% ruthenium to add hardness and durability.
Palladium has a slightly darker hue than platinum, edging closer to silvery-gray than silvery-white. For practical purposes, though, palladium is visually indiscernible from platinum. A trained professional would have to closely examine these two platinum metals side by side to tell the difference.
As well as palladium being harder and more scratch-resistant than platinum, it’s also about fifteen times as rare, which contributes to its higher price point. At the time of updating this guide, palladium is about 30% more expensive than platinum.
However, because it’s less dense, palladium jewelry uses less metal by weight than jewelry made from platinum. As such, palladium jewelry isn’t significantly more expensive than platinum jewelry.
Palladium Quick Facts Chart
|Chemical Symbol||Pd||Mohs Hardness||4.75 to 5|
|Density||6.95 oz per cubic inch||Year Discovered||1803|
|Melting Point||2,831 °F (1,555 °C)||Price per Oz||$1,260 USD (as of Aug 2023)|
Rhodium is a very rare, extremely white platinum group metal that, in the jewelry industry, is used only for electroplating. This works by suspending a piece of jewelry in a liquid rhodium solution and passing an electric current through it, which electrically bonds a very thin layer of rhodium to the jewelry.
Electroplating is probably familiar to anyone who has bought white gold jewelry in recent years. That’s because nearly all white gold jewelry is now plated with rhodium. This is done to make it whiter, by covering up the natural yellow tint inherent to white gold.
Sterling silver is also sometimes rhodium-plated, and even platinum can get a coat of rhodium before hitting a store’s jewelry display. As these metals are both already naturally white, rhodium plating may not necessarily improve their appearance, but it will still protect them against discoloration and scratching.
Rhodium plating can also be dark instead of light. “Black rhodium” is achieved by mixing powdered rhodium with darkening additives. When electroplated onto jewelry, this darkened coating can appear from medium gray to almost-black in color. This is often described as a gunmetal finish, or sometimes as “black gold“.
Since rhodium is so popular for electroplating, why don’t jewelers simply make solid rhodium jewelry? One reason is that the element is difficult to work with, having both a high melting point and low malleability. Another reason is that rhodium is spectacularly expensive – currently valued at more than four times as much as platinum!
Fortunately, rhodium plating does not significantly increase the price of jewelry, since the coating is microscopically thin. But the downside of that thinness is that the plating will wear off over time. For instance, a ring subjected to daily wear will need to be replated every one to two years, at a cost of around $60 to $120 per treatment.
Rhodium Quick Facts Chart
|Chemical Symbol||Rh||Mohs Hardness||6|
|Density||7.17 oz per cubic inch||Year Discovered||1803|
|Melting Point||3,565 °F (1,963 °C)||Price per Oz||$4,200 USD (as of Aug 2023)|
Though ruthenium is currently only about half the price of platinum, solid ruthenium jewelry is extremely difficult to find. Compared to platinum and even palladium, this platinum group metal has low recognition and low demand in the jewelry market.
One reason for this is that it’s is too hard and brittle for jewelers to easily work with. Another may be that it’s slightly darker than platinum, so jewelry made from a mostly ruthenium alloy would appear less bright than jewelry made from an alloy of mostly platinum.
However, ruthenium is often added in small amounts to both platinum and palladium jewelry alloys. A platinum or palladium alloy with 5% ruthenium content added is harder and more scratch-resistant than pure platinum or pure palladium alone.
Jewelry can also be plated with ruthenium, which gives it a glossy finish that protects against scratching and tarnishing. As with rhodium plating, this can provide either a bright white finish, or a dark one. If the jewelry features diamonds or other white gems, a dark coating can make them appear brighter and sparklier by contrast.
Important safety note: Ruthenium reacts with common household bleach to form toxic compounds. You should never wear jewelry that contains this element while cleaning with or otherwise handling bleach, in case of accidental exposure.
Ruthenium Quick Facts Chart
|Chemical Symbol||Ru||Mohs Hardness||6.5|
|Density||7.17 oz per cubic inch||Year Discovered||1844|
|Melting Point||4,231 °F (2,333 °C)||Price per Oz||$465 USD (as of Aug 2023)|
In its pure element form, iridium is silvery-gray with a faint yellow tinge. It’s one of the hardest and rarest precious metals, as well as being the second-densest natural element.
Iridium is also very brittle, requires huge amounts of heat to melt, and currently trades at more than four times the price of platinum. These factors make iridium an unpopular metal with jewelers, thus most are not skilled in crafting with it.
Smithson Tennant is perhaps the only company that’s currently producing and marketing solid iridium jewelry. You can see in the publicity images on their website that their iridium rings do have a faint yellowish cast, akin to unplated white gold.
Iridium is more well-known for its use in platinum jewelry. Platinum-iridium alloys contain mostly platinum, and between 5% and 10% iridium. The iridium content hardens the platinum, making it less likely to dent or scratch, but the amount is low enough for the alloy to remain malleable.
Iridium Quick Facts Chart
|Chemical Symbol||Ir||Mohs Hardness||6.5|
|Density||12.96 oz per cubic inch||Year Discovered||1803|
|Melting Point||4,435 °F (2,446 °C)||Price per Oz||$4,500 USD (as of Aug 2023)|
Osmium shares characteristics with its fellow PGM iridium: both are very dense and brittle, and silvery-gray in color. Except where iridium has a faint yellowish cast, osmium carries a slight tint of blue. Like iridium, osmium is a hard metal with an extremely high liquefaction point.
However, unlike iridium and the other platinum group metals, osmium can be dangerous in its pure form. Though osmium is not itself toxic, it can oxidize at room temperature to form osmium tetroxide – a highly toxic compound.
Despite this, it’s still possible to use osmium as a jewelry metal. When pure osmium is transformed into crystallized osmium, it no longer oxidizes upon contact with air, and so does not pose any health hazard. As well as serving as a jewelry metal, crystallized osmium can also be used to create faux gemstones that resemble diamonds.
Still, osmium jewelry remains scarce. Its extreme rarity, dangerous reputation, difficulty to work with, and the laborious process required to crystallize it, have so far discouraged the jewelry market from embracing osmium.
The price of osmium remains low relative to most other platinum group metals. Not only is there little demand for it in the jewelry industry, osmium has limited practical applications elsewhere. By contrast, the other platinum metals are in high demand for their chemical, automotive, and other industrial uses.
Osmium Quick Facts Chart
|Chemical Symbol||Os||Mohs Hardness||7|
|Density||13.06 oz per cubic inch||Year Discovered||1803|
|Melting Point||5,491°F (3,033 °C)||Price per Oz||$400 USD (as of Aug 2023)|
If you found this article about the platinum group metals informative, you may also enjoy learning the answers to some frequently asked questions about platinum jewelry.