Gold, the traditional "noble metal" used in jewelry for centuries, is also extremely popular in crafting the cases of watches. Typically, when one thinks of gold, it is the yellow variety that springs to mind. However, other shades of gold have also been employed in jewelry making, and these variations are highly prized as well. The purity of gold used in watch cases has varied over the years; while 14-Karat gold was once fairly widely used, the vast majority of today's fine watches use well-alloyed 18K gold which retains a beautiful luster due to the high gold content, while remaining adequately durable for daily wear.
All forms, or subspecies, of 18K gold have an equal gold content, which amounts to a 75% level of pure gold, along with 25% alloy mixed in. Expressed another way, 24K gold is 99.999% pure gold, or the closest available thing to pure, elemental gold---thus, 18 karat purity denotes a content of 3/4 pure elemental gold. By contrast, 14 karat gold contains 58% gold, while the remaining 42% is alloy.
The metals which comprise the alloy determine the color of the gold. 18K yellow gold is composed of 75% gold, 16% silver and 9% copper. By reversing the amounts of silver and copper, 18K rose gold (also known as pink gold) results, which contains 16% copper and 9% silver respectively. 18K white gold contains 75% gold, 3% silver, 9.5% copper, and approximately 12.5% palladium. Palladium is in the platinum "family" of metals, and is largely responsible for imparting the greyish-white color to white gold.
The percentages listed above are all approximate, however, and the admixture of alloy can vary by manufacturer. This is why some rose gold watches seem "rosier" than others, while some yellow gold cases have a less brassy tone than others. Further variants can be created by altering the alloy composition in other ways. For example, by adding even greater copper content to rose gold, and correspondingly reducing the silver content even further, a ruddier subspecies of rose gold results which is sometimes called "red" gold. The elegant creations by Jaquet Droz are an example of watches which use red gold for their cases.
Yellow gold, too, is sometimes altered to obtain a slightly different color, by increasing the silver content and reducing copper. The result of this combination is nicknamed "green" gold due to the slight color change imparted by the higher silver content. This color is infrequently used in making watch cases, although some manufacturers, such as Franck Muller have used it for certain pieces.
It is worth noting that many 18K white gold watch-cases (and jewelry pieces) are given a plating of rhodium. Rhodium does not occur naturally, but is a bi-product of platinum production, and is extraordinarily difficult to obtain. However, due to it's brilliance, and hard finish, it is used to plate the white gold, which has a slightly greyish to greyish-brown tone. When such a watch is refinished, the old rhodium coating is stripped completely off the gold and a new layer is applied. This is done because new rhodium cannot be applied to a rhodium surface, as it will not bond properly.
It should be noted that not all manufacturers use rhodium plating for their 18K white gold. Blancpain for example, uses an 18K gold alloyed with a very high percentage of palladium. This gives the metal a whiter tone than would ordinarily be produced, without having to resort to the use of rhodium. This makes refinishing somewhat easier as there is no plating to have to reapply to the metal surfaces.
Trends come and go, and while the various colors of gold vie for supremacy in the eye of the luxury buying public, one thing remains constant: the demand for gold. Despite challenges from exclusive and rare metals such as platinum,which has a considerably higher dollar per ounce cost than gold, and high-tech newcomers like titanium and aluminum, gold remains the classic--and penultimate--metal for the luxury watch and jewelry industry.