Short answer
A replica Rotonde 18k is a copy of a Cartier Rotonde de Cartier watch that a seller describes as 18-karat gold. The phrase carries two claims. One is the model. The other is the metal. Model copies are common. Solid 18k gold cases in replicas are not. Most watches sold under this phrase have a gold-plated steel case.
What Cartier sells under the Rotonde name
Rotonde de Cartier is a round case line. Cartier builds it in 18k pink gold, 18k yellow gold, 18k white gold, and platinum. Case diameters run from about 40 mm to 47 mm, set by model. Movements include the Cartier caliber 1904 MC and the 1904-CH MC chronograph. Retail prices for gold Rotonde models start near $20,000 and pass $100,000 for tourbillon and mystery-clock versions.
What 18k means on a case
18k gold holds 75% gold by weight. The other 25% is copper, silver, palladium, or zinc. US rules in the FTC Jewelry Guides allow an "18K" or "750" stamp when the alloy meets that standard. A 40 mm solid gold case weighs about 50 to 80 grams. Gold spot price moves each day, so the raw metal in that case costs thousands of dollars.
How replica sellers use the term
- Gold plating. A layer of gold over steel or brass. Thickness runs from a few microns to about 20 microns.
- Gold filled. A bonded layer of gold alloy over a base metal. Marked "18k gold filled" or "18k GF".
- Vermeil. Gold over sterling silver. Common in jewelry, rare in watch cases.
- Solid 18k. The full case, and often the bracelet, is 18k. Rare in the replica market and priced in four figures.
Ask the seller for case weight and for a photo of the hallmark inside the caseback. A seller that will not state the weight has not sold a solid gold case.
Tests a buyer can run
X-ray fluorescence (XRF) reads a few microns deep. A plated case can pass this test. An acid test on a scratch inside a lug reads deeper. A density test with water displacement gives the clearest result: 18k gold runs about 15.5 g/cm3, steel about 7.9, brass about 8.5. Gold is not magnetic, so a strong magnet that pulls on the case shows the base metal is not gold.
Legal position in the US
19 U.S.C. § 1526 bars import of goods that carry a counterfeit mark. CBP seizes such shipments and can destroy them. Selling a counterfeit watch as a Cartier product carries civil and criminal exposure. A seller who stamps "18K" on a plated case breaks FTC marking rules on top of trademark law.