Carats in gold and carats in gemstones are two different units of measurement, and confusing them is one of the most common mistakes even among those working in the industry. In watchmaking, we use both, and it's worth keeping them separate with precision.
For gold, the carat expresses purity as a fraction of 24: 18-karat gold contains 18 parts of pure gold out of 24, or 75%, and is also marked as 750 or Au750. 14-karat is 58.3%, 9-karat is 37.5%, as indicated by the Au375 marking visible for example on Omega's Bronze Gold. 24-karat gold is pure gold, too soft for any structural application in watchmaking. For gemstones, instead, the carat is a unit of weight: one carat equals exactly 200 milligrams. A 9.78-carat Paraiba tourmaline weighs almost two grams, regardless of its mineralogical purity.
In practical work at the bench, what really matters for watchmaking gold is not only the percentage of pure metal, but the composition of the remaining elements. 18-karat alloys can differ profoundly in behavior: hardness, wear resistance, and color stability over time depend on what occupies that 25% that is not gold. Copper raises the tone toward red and hardens the alloy; silver lightens it; palladium improves oxidation resistance. IWC in its Armor Gold declares a modified internal microstructure to increase hardness without changing the golden proportion. Omega in Moonshine Gold balances silver, palladium, and reduced copper to achieve a cooler tone and stability over time. A. Lange & Söhne in Honeygold achieves an alloy harder than platinum itself, with a tone between yellow and rosy that is very subtle.
There are also rarer 21-karat alloys, used for example by Andersen Genève in BlueGold guillochè: the higher golden percentage leaves less room for secondary elements, and the characteristic color derives from a surface treatment, not from a base oxide. A 21-karat gold worked by hand requires sharp tools and steady hands: it's softer than standard 18K precisely because it has less copper.
In watchmaking, gold also enters the moving parts of the movement. Lang Heyne uses solid 14-karat gold gears in the Calibre VI: the friction between gold and steel pivots is less than between steel and steel, with a real benefit to efficiency. It's not decoration: it's documented technical choice.