Short answer

A genuine Omega Seamaster Diver 300M has a 42 mm steel case, a ceramic dial with laser-cut waves, a date window at 6 o'clock, and a conical helium escape valve at 10 o'clock. The case back is sapphire. The movement is the Co-Axial Master Chronometer calibre 8800, certified by METAS. Fakes miss these details in combination, not one at a time. A photo cannot confirm a watch. Only a watchmaker with the case back open can.

Case and size

• Diameter 42 mm. Thickness about 13.6 mm. Lug width 20 mm.
• Steel, titanium, or gold. The steel blue-dial model is reference 210.30.42.20.03.001.
• Helium escape valve at 10 o'clock. Conical, screw-down, marked with the He symbol.
• Crown at 3 o'clock, conical, signed with the Omega symbol, guarded by two steel shoulders.
• Lugs are twisted. The side profile is not flat.

Dial and bezel

• Ceramic dial with a laser-engraved wave pattern. The waves sit below the surface, not printed on top.
• Applied metal Omega logo. It stands off the dial.
• Skeleton rhodium-plated hour and minute hands. The seconds hand carries a lollipop dot with lume.
• Date at 6 o'clock in a beveled aperture.
• Ceramic bezel insert with white enamel or Liquidmetal markings and a scalloped steel edge.
• Lume pip at 12 on the bezel. Glow color changes with the reference, so compare against the same reference number, not against a photo from another model.

Movement and case back

• Calibre 8800, automatic, 55-hour power reserve, 25,200 vibrations per hour, 35 jewels.
• Co-Axial escapement and silicon balance spring. Anti-magnetic to 15,000 gauss.
• METAS Master Chronometer card. Daily rate 0 to +5 seconds.
• Sapphire case back. Rotor with Geneva stripes and Co-Axial Master Chronometer text.

Most fakes use an off-the-shelf three-hand movement. There is no Co-Axial escapement and no silicon spring. A magnet test is one signal. A genuine Master Chronometer keeps its rate after contact with a strong magnet. A standard movement runs fast after the same contact.

Numbers and paperwork

• Serial number engraved on the case. A movement number sits inside. On a genuine watch the two match.
• Reference number format: 210.30.42.20.03.001. The last three digits show dial and bracelet.
• Warranty card with reference and serial printed on it. Omega can check a serial against its records.

What a photo cannot show

Case polish, dial print, and hand shape can be copied to a close level. A photo cannot show the escapement, the balance spring, or the case back threads. Book time with a watchmaker. Ask for the case back to be opened and the movement photographed.

Price signals

Genuine steel Diver 300M models sell new above 5,000 USD and hold resale value. Offers far below the current Omega price list are a signal, not proof. Prices move, so check the current list before you compare.