In watchmaking, bimetal refers to a case, bracelet, or component constructed from two distinct metals that produce a visible chromatic contrast. The most common combination is stainless steel and 18-carat yellow gold; followed by steel with rose gold and, less frequently, white gold with yellow gold. Rolex has codified this approach under the commercial name Rolesor: Yellow Rolesor unites Oystersteel and 18k yellow gold, Everose Rolesor replaces yellow gold with the proprietary Everose alloy. Patek Philippe instead uses alphanumeric reference codes to distinguish alloys, such as the R/G pairing in ref. 5520RG where the case is in rose gold and the details in white gold.
The constructive logic of bimetal is not purely aesthetic. In integrated bracelets, the center links in gold and the outer links in steel respond also to a matter of weight and cost: gold softens the visual structure without making it excessively heavy. The junction between the two metals requires attention to galvanic compatibility; poorly executed assembly can accelerate contact corrosion, especially in the presence of salt sweat.
The concept extends to less conventional metals. Tantalum, with its density comparable to platinum and its characteristic blue-gray hue, is occasionally paired with steel or titanium; its machining is problematic because hardness can damage finishing equipment. Niobium, chemically akin to tantalum, offers a lighter gray and slightly better workability. Steel with a coating of platinum group metals — rhodium, palladium — falls into a hybrid category: technically a single metal with plating, not a true bimetal, but the surface effect can simulate an artificial aging patina as in the Edox SkyDiver aged steel.
An extreme variant of bimetal is Mokume-gane, a seventeenth-century Japanese technique that joins alternating layers of non-ferrous metals — silver, gold, copper — through solid-state diffusion until reaching a homogeneous stratified block. By etching and deforming the block, wood-like grain patterns emerge. In contemporary watchmaking it is applied mainly to dials and, more rarely, to cases. Kees Engelbarts was among the first to introduce it to dials; Ondřej Berkus uses it on silver and copper cases.
From an environmental perspective, the extraction and refining of precious metals involves high-temperature processes and toxic substances. The supply chain is often concentrated in countries with less rigorous controls. Some maisons, including Piaget which has operated an in-house foundry for nearly eighty years, have chosen to internalize processing also for reasons of raw material traceability.