Everyone’s heard of platinum, but far fewer people know that it’s just one of six precious metals classed as platinum group metals (PGMs). The other five are palladium, rhodium, ruthenium, iridium, and osmium.
The platinum group metals share similar appearances and physical properties. They all have a lustrous silvery color, are resistant to tarnish and corrosion, require high temperatures to melt, and are typically found close together in nature.
Read on to find out more about the characteristics of each PGM, and how it is used in the jewelry industry.
Platinum is the eponymous and most abundant member of the platinum group. Prized for its silver-white sheen, rarity, and resistance to tarnish and corrosion, it has been used both as currency and in the making of platinum jewelry for hundreds of years.
Much of the value of platinum derives from its extraordinary scarcity. It is 30 times rarer than gold, and as such, is usually priced higher. However, the prices of precious metals are always fluctuating, and at the time of updating this guide, gold is the more expensive of the two.
High density is another notable characteristic of platinum. It 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 also 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 is 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, popular platinum alloy formulations for jewelry include:
- 95% platinum and 5% ruthenium
- 95% platinum and 5% cobalt
- 95% platinum and 5% iridium
- 90% platinum and 10% iridium
These formulations show that platinum jewelry is typically of very high purity – 90 to 95% of the metal content is pure platinum. By comparison, gold jewelry alloys are typically 75% pure (18 karat gold) or 58.3% pure (14 karat gold).
Platinum Quick Facts
|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||$1,010 USD (as of Jan 2022)|
Apart from platinum, palladium is the only one of the platinum group metals used in large amounts within jewelry metal 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 silver-gray than silver-white. For practical purposes, though, palladium is visually indiscernible from platinum. A trained professional would have to closely examine these two PGM metals side by side to tell the difference.
As to other physical differences, palladium is 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 (January 2022), palladium is more than twice as expensive as platinum.
However, because it is less dense, palladium jewelry uses less metal by weight than jewelry made from platinum. As such, palladium jewelry is currently priced only slightly higher than platinum jewelry, rather than twice as high.
Palladium Quick Facts
|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||$2,400 USD (as of Jan 2022)|
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 tinge 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 rhodium 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 rhodium is difficult to work with, having both a high melting point and low malleability. Another reason is that rhodium is a spectacularly expensive PGM – currently valued at about 16 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 rhodium 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
|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||$16,700 USD (as of Jan 2022)|
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, solid ruthenium has low recognition and low demand in the jewelry market.
One reason for this is that ruthenium is too hard and brittle for jewelers to easily work with. Another may be that ruthenium is 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 to improve their durability. A platinum alloy with 5% ruthenium content is harder and more scratch-resistant than 100% platinum. Adding 5% ruthenium gives the same advantages to a palladium alloy.
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 black ruthenium coating can make them appear brighter and sparklier by contrast.
An important safety note to keep in mind – ruthenium reacts with common household bleach to form toxic compounds. You should never wear ruthenium-plated or ruthenium-alloyed jewelry while cleaning with or otherwise handling bleach, in case of accidental exposure.
Ruthenium Quick Facts
|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||$530 USD (as of Jan 2022)|
In its pure element form, iridium is silvery-gray with a faint yellow tinge. It is 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 five times the price of platinum. These factors make iridium an unpopular metal with jewelers, thus most are not skilled in crafting with it.
At the time of updating this guide, Smithson Tennant is perhaps the only company that’s producing and marketing solid iridium jewelry. You can see in the publicity images on their website that their iridium rings do have a slight 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
|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||$3,950 USD (as of Jan 2022)|
Osmium shares characteristics with its fellow PGM iridium: both are very dense and brittle, and silver-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 melting 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 rare. Its extreme rarity, dangerous reputation, difficulty to work with, and the laborious process required to crystallize it, have so far discouraged most jewelers 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 PGM metals are in high demand for their various uses in the chemical, automotive, and electronics industries.
Osmium Quick Facts
|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 Jan 2022)|
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.