Short answer
Almost every watch sold as a fake Clé de Cartier in gold is a base metal case with gold plating, not solid gold. Solid 18k replicas exist, but they are rare, and the people who make them usually price them near a used genuine piece. So ask a narrower question: is this plating, and how well does it hide it? The Clé de Cartier arrived in 2015, named for its key-shaped crown, and the gold versions came in 31mm, 35mm and 40mm cases with Cartier's in-house 1847 MC movement. A replica has to fake all of that. Weight is where the fakery shows first.
Weight and balance
Yellow gold runs about 15.5 grams per cubic centimeter, so a 35mm Clé on a gold bracelet lands well past 130 grams. A plated steel copy of the same dimensions sits closer to 90 or 100 grams, and a hollowed brass one can feel lighter still. The gap is not subtle once you have held a genuine piece. Plated copies also carry their mass in the bracelet and almost none in the head, which makes them sit differently on the wrist.
Hallmarks and stamps
Swiss gold carries an official hallmark plus a purity stamp, and 18k reads as 750. On a real Clé you find those marks inside the caseback, on the clasp and on the back of the lugs, along with a maker's mark and a serial. Replicas either skip the stamps or engrave them too deep and too even, in a font that sits a hair off. If the serial on the caseback disagrees with the one on the card, stop there.
The crown is the whole point
The Clé crown is cut to look like a key head and it turns to set the watch, with a sapphire cabochon at the end. On most replicas the key shape is stamped rather than milled, the cabochon is glass and sometimes glued, and the crown pulls out like an ordinary watch instead of seating flush. Run a fingernail along the crown edges. Crisp machining is a good sign, rolled or soft edges are not.
Crystal, dial and date
Sapphire beads water into tight drops. Mineral glass, which most copies use, lets the drop spread. Check the date window too: the genuine aperture is cut clean at the edges and the numerals are sharp, while replicas show a fuzzy ring around the opening and a date font that is a touch too wide. Blued steel hands on a gold case should look dark blue, not painted purple.
Movement and caseback
The 1847 MC beats at 28,800 vibrations per hour with roughly 40 hours of reserve, and the rotor winds in both directions. Fakes use a Miyota 9015 or a Chinese clone. The engraving on the caseback looks sandblasted rather than laser sharp, and the movement decoration is printed instead of applied. Listen for a rattling rotor when you tilt the watch.
Plating versus solid gold
Decent replicas carry 2.5 to 5 microns of plating. That wears through at the crown, the lugs and the clasp edges within months of daily wear, and once the base metal shows, the watch is done. Three checks settle it:
- XRF scan or acid test at a jeweler, done on an inconspicuous spot
- Magnet test, since 18k gold is not magnetic and plated steel is
- Wear pattern on high-contact edges
One legal note worth knowing. Selling counterfeit goods is trademark infringement in the US, and Customs can seize counterfeit imports. Owning one is a personal call; reselling one is a real risk.