Rhodium is a silvery-white metal used primarily as a plating layer on jewelry, particularly white gold and sterling silver, to enhance brightness and durability. Rhodium does not turn green; it is highly resistant to discoloration. If rhodium-plated jewelry shows a green hue, it signifies that the ultra-thin rhodium layer has worn away. The green color indicates the exposure and subsequent reaction of the base metal underneath, an issue separate from the rhodium itself.
The Chemical Stability of Rhodium
Rhodium is classified as a noble metal, placing it in the same group as platinum, palladium, and gold, characterized by inherent stability. This stability means rhodium does not react with common environmental factors like air or moisture. Because of this inertness, rhodium is extremely resistant to both oxidation and corrosion, the chemical processes that cause many other metals to tarnish or change color.
The metal’s high melting point, reaching 1964°C, further contributes to its durability and resistance to chemical change. Rhodium is also highly resistant to most acids and alkaline solutions, making it an excellent protective barrier. These properties ensure that when the plating is intact, it remains bright, reflective, and chemically unaltered, maintaining its silvery-white appearance.
Why Rhodium-Plated Items Change Color
The green discoloration on rhodium-plated items is a symptom of the plating wearing off, not a failure of the rhodium itself. Rhodium is applied as a microscopic layer, often between 0.75 and 1.0 microns thick, onto a base metal, such as copper, brass, or a white gold alloy containing copper and nickel. This base metal is the true source of the color change.
When the thin rhodium coating wears through due to friction or chemical exposure, the underlying metal is exposed to air and moisture. Metals like copper react with oxygen, salts, and acids, particularly those found in human sweat and skin oils. This chemical process, known as oxidation, forms copper carbonate, a blue-green substance also known as verdigris.
The verdigris compound transfers to the skin or remains on the jewelry surface, creating the noticeable green mark. Skin chemistry and environmental factors accelerate this reaction; individuals with more acidic skin or those who sweat heavily may experience the greening effect faster. The presence of nickel in some base alloys can also contribute to discoloration, but the bright green color is predominantly caused by copper oxidation.
Maximizing the Lifespan of Rhodium Plating
Since the green color only appears after the plating has worn away, extending the rhodium layer’s life is the best way to prevent discoloration. Minimizing physical abrasion is the most effective strategy, as constant rubbing is the primary cause of plating wear. Remove rhodium-plated rings and bracelets before engaging in manual tasks, exercise, or activities where the jewelry might rub against hard surfaces.
Exposure to certain chemicals can rapidly degrade the rhodium layer. Avoid contact with harsh cleaning products, household chemicals, and chlorine, such as that found in swimming pools, as these accelerate the breakdown of the metal. Apply cosmetics, perfumes, and lotions and allow them to dry fully before putting on any plated jewelry.
For cleaning, use only a mild solution of soap and lukewarm water, followed by a gentle rinse and thorough drying with a soft, lint-free cloth. Avoid using abrasive cleaners or ultrasonic machines, which can strip the thin plating. When storing pieces, keep them separate in a soft pouch or a lined jewelry box to prevent scratching.
Rhodium plating is not permanent and will eventually wear down, typically lasting between six months and two years for high-contact items like rings. Once the base metal begins to show through, the only way to restore the original appearance and protection is to have the piece professionally re-plated. Regular maintenance and timely reapplication of the rhodium layer are necessary to keep the jewelry bright and prevent the underlying metal from oxidizing.