Quick Answer
A fake Jaeger-LeCoultre Master in gold fails three checks. First, the case has no Swiss assay hallmark and no 750 fineness stamp. Second, the weight is wrong, because 18k gold has a density near 15 g/cm3 and brass sits near 8.5 g/cm3. Third, the movement is generic. A real Master Control automatic uses a JLC calibre with Geneva stripes, blued screws, and an engraved rotor. A copy hides a plated rotor over an ETA, Seagull, or Miyota base.
Hallmarks and the 750 Stamp
Swiss law requires an official assay hallmark on gold watch cases sold in Switzerland. A genuine Master caseback shows two marks next to the reference number: the fineness stamp 750 or Au750, and the assay office hallmark. Both are struck into the metal. A laser engraving that sits on top of the surface is a warning sign.
The buckle and the deployant clasp carry the same stamps. So does the oscillating weight on models that use a solid gold rotor.
Weight and Density
- 18k yellow gold: about 15.5 g/cm3
- 18k rose gold: about 15.0 g/cm3
- 18k white gold: about 14.7 g/cm3
- Brass: about 8.5 g/cm3
- 316L steel: about 7.9 g/cm3
A 37 mm gold Master Control case weighs more than double a brass copy of the same size. Plated brass feels light in the hand.
Two tests measure this. A jeweler can run XRF in about two minutes and it leaves no mark. A density test needs the case back off. An acid test leaves a visible spot, so it damages resale value.
Magnet Tests Do Not Work
Gold is not magnetic. Brass is not magnetic. 316L steel is close to non-magnetic. A magnet tells you nothing about a watch case. Skip this test.
Movement and Rotor
Master Control automatics run the JLC calibre 899 family. The hand-wound Master Ultra Thin runs the calibre 849. Both show Geneva stripes on the bridges, perlage on the base plate, blued screws, and the JLC name plus the calibre number engraved on the rotor.
Copies hide a plain movement under a decorated plate or a gold-plated rotor. Look at the balance wheel, the regulator, and the engraving depth. Factory engraving cuts clean lines with square edges. Copy engraving looks shallow and soft.
Dial, Case, and Strap
Gold Master models use applied gold indices and gold hands. Printing is flat and sharp: the brand name at 12 and Swiss Made at 6. A dot or a smudge in the print points to a redial or a copy.
Original straps are alligator with a signed gold buckle. Many fakes ship with embossed calf leather and a steel buckle with gold color.
Papers, Serial Numbers, and Price
The caseback reference and serial must match the warranty card. Ask for a JLC service record. A watchmaker can check the serial against the factory archive.
Gold content sets a price floor. A case and buckle with 30 g to 40 g of 18k gold holds 22.5 g to 30 g of fine gold. At a spot price of $2,400 per ounce, that is about $1,700 to $2,300 in metal alone. Spot prices change each day, so check a current quote. Any 18k Master sold far below that figure is plated.
Limits of Home Tests
XRF reads the surface layer. A plated case can read as 18k on the outside. Test the inside of the caseback with the movement removed.
Tungsten has a density of 19.25 g/cm3, which sits above gold. A tungsten core can imitate the weight of a gold case. Weight checks raise questions but do not settle them.
The final step is a service center. Send the watch to JLC or an authorized watchmaker. They open the case, read the serial, and compare the movement against factory records.