Short answer
Fake Rotonde 18k is a listing phrase, not a model. It points to a copy of a Cartier Rotonde de Cartier watch that the seller describes as 18k gold. Three claims sit inside the phrase: the Cartier model name, the copy status, and the gold content. Each can be checked on its own. In most listings that use this wording, the case is gold plated brass or steel, not solid 18k gold.
What 18k gold means
18k gold is an alloy of 75 percent gold and 25 percent other metals such as copper, silver, zinc or palladium. Pure gold is 24k, so 18k equals 18/24, or 750 parts per thousand. Accepted stamps include 750, 18K, 18k and Au750.
Swiss law covers gold watch cases made in Switzerland. An official assay office applies the hallmark. The Swiss hallmark for 18k gold is the St. Bernard dog head. France uses the eagle head for 750 gold. US rules come from the FTC Jewelry Guides, which bar the word gold on items that are not solid gold.
A stamp proves little by itself. Stamping tools cost less than a gram of gold, and counterfeit cases carry fake stamps. X-ray fluorescence or acid assay is the real test.
Solid gold, gold filled, gold plated
- Solid 18k: the full case is the alloy. Density is about 15.5 g/cm3. Stainless steel is about 7.9 g/cm3. A solid 40 mm gold case weighs close to twice a steel case of the same size.
- Gold filled: a gold layer bonded to base metal. US rules require the layer to be at least 1/20 of the total weight for the gold filled label.
- Gold plated: a thin electroplated layer. Watch plating runs from about 0.5 to 5 microns. Sellers quote 5 microns as a premium figure.
Five microns over a 40 mm case holds under a gram of gold. Scrap value stays small. A solid 60 g 18k case holds 45 g of gold, and gold trades at a market price that moves each day.
The Rotonde de Cartier line
Cartier uses Rotonde de Cartier for round cases. Models include the Rotonde de Cartier Astrotourbillon, the Astrocalendaire, the Mysterious Hour and the Chronograph. Case sizes run from about 40 mm to 47 mm across the line. Cartier builds some references in steel and others in 18k gold, white gold or platinum, so the model name alone does not mean gold. Each reference has its own number, and that number identifies the metal.
Checks that work
- Read the caseback. Look for a reference number, a case number and Swiss Made.
- Weigh the watch. Gold and steel differ by a factor near two.
- Find the hallmark. On Swiss cases it sits on the caseback or between the lugs.
- Ask for an assay. XRF reads the alloy, not the plating.
- Skip the magnet test. It fails on both gold and most stainless steel.
US law
18 U.S.C. 2320 makes trafficking in counterfeit goods a federal crime. A first offense carries up to 10 years in prison. Customs and Border Protection can seize shipments under 19 CFR Part 133. The FTC Jewelry Guides require accurate wording on gold content in ads and labels. Selling a copy as a genuine Cartier watch is trademark infringement. The buyer gets no warranty and no service support from Cartier.
Unknowns
Cartier does not publish production totals for the Rotonde line. The number of copies in the resale market is not known. A listing that calls a copy 1:1 or same as original makes a claim that cannot be verified from the outside.