Short answer
A fake Omega Seamaster is a counterfeit watch made to look like a genuine Seamaster. It copies the case, dial, bezel, and caseback markings. It does not use an Omega movement or Omega materials. A counterfeit fails inspection at an Omega service center.
Check the reference number and serial
A genuine Seamaster has a reference number. For the Seamaster Diver 300M, the reference starts with 210.30. Example: 210.30.42.20.03.001. The same number appears on the warranty card, hang tag, and invoice. A fake may show a reference that does not match the dial color or bracelet.
How to Identify a Fake Omega Seamaster
Find the serial number. Omega engraves it on the caseback, on a lug, or on the movement. Compare the caseback serial with the movement serial. A mismatch points to a counterfeit.
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Inspect the caseback and movement
Many Seamaster models have a sapphire caseback. You can see the rotor and the balance wheel. Genuine Master Chronometer calibers 8800 and 8900 have a rotor with the Omega logo, Co-Axial text, and a serial number. The caliber 8800 runs at 25,200 vibrations per hour and has a 55-hour power reserve. A timegrapher can confirm the beat rate.
A fake may use a solid caseback, a printed metal plate, or a third-party movement. A printed plate does not move. A genuine balance wheel oscillates.
Look at the dial and bezel
The Seamaster Diver 300M has a ceramic dial with laser-engraved waves. The date window sits at 6 o'clock. The helium escape valve sits at 10 o'clock. The bezel insert is ceramic with an enamel or Liquidmetal scale. A fake may show printed waves, a painted bezel insert, a flat helium valve, or a thick date font.
Genuine indexes are applied metal with Super-LumiNova. A fake may use printed markers. Lume glow is not proof. Check the alignment of the bezel triangle with the 12 o'clock marker.
Measure the case
The Seamaster Diver 300M case is 42 mm wide and 13.6 mm thick. The crown screws down. The helium valve has threads. A fake may use a case that is 1 mm wider or thinner. Measure the case with calipers. Compare the result with the reference specification.
Check the price and seller
A new Seamaster Diver 300M lists above 5,000 USD at retail. A price far below market is a warning sign. Ask for the original invoice, a dated warranty card, and the box. Meet at an Omega boutique or a watchmaker for inspection. Pay with a method that allows a chargeback.
What to do with a suspect watch
Take the watch to an Omega service center. A watchmaker can open the case and check the movement. Counterfeit sales are illegal in the US. Report a seller to the FTC and to U.S. Customs and Border Protection.