Short answer
Blancpain builds the Fifty Fathoms in 18k white gold. That metal is gold alloyed with palladium or nickel, then plated with rhodium. Replica versions use stainless steel cases with a rhodium, palladium, or white gold tone plating. No replica case is solid gold. The gap shows up in three places: weight, hallmarks, and price.
Which reference this covers
The name covers more than one watch. The main line is the Fifty Fathoms Automatique, reference 5015. Case size is 45 mm. Water resistance is 300 m. The movement is caliber 1315, an automatic with three mainspring barrels and a 120 hour power reserve. Blancpain has sold the 5015 in steel, titanium, red gold, and white gold. The Fifty Fathoms Bathyscaphe comes in 38 mm and 43 mm, and white gold versions exist in that line too. Reference numbers change with dial and strap, so dealers confirm the exact number from the case back.
What 18k white gold means
18k means 75 percent gold by weight. The rest is alloy. White gold takes its color from palladium, nickel, or silver in the mix. Most white gold cases carry a rhodium plating for a bright finish. That plating wears at the edges over years, and the warmer tone under it shows. Density of 18k gold sits near 15.5 g/cm3. Steel sits near 7.9 g/cm3. The difference is why a solid gold dive watch feels heavy in the hand.
How replicas of this model are made
Most replica Fifty Fathoms watches use a 316L or 904L stainless steel case. The factory then applies a plating. Common finishes are rhodium, palladium, and white gold tone PVD. Plating thickness is often under 5 microns. Some sellers list terms like "18k gold plating" or "18k wrapped." Both mean a thicker layer, not a solid gold body. A solid 18k case on this model would hold thousands of dollars in metal at current gold prices. No watch at replica prices has one.
Checks that separate plating from solid gold
- Weight. A 45 mm Fifty Fathoms on a strap sits near 130 g in steel. A plated replica lands in the same range. Solid 18k gold sits well above that.
- Hallmarks. Swiss law requires a precious metal stamp and a maker mark on gold cases. Look for the 750 stamp, the St. Bernard dog head, and the case maker punch. Plated replicas have none, or have marks that do not match.
- Case back. Reference, serial, and metal code are stamped. White gold codes differ from steel codes.
- Bezel insert. The 5015 uses a sapphire insert with lume under it. Replicas use sapphire or coated glass. Lume brightness and color vary.
- Movement. Most 5015 Automatique versions show caliber 1315 through a sapphire case back. Replica movements are Miyota 9015, Seagull, or Hangzhou clones with a decorated rotor. Beat rate and balance wheel position differ.
- Finish tone. Rhodium plating holds a cold white color. Solid white gold shifts warmer as plating or polish wears.
Water resistance and service
Blancpain rates the Fifty Fathoms at 300 m. Replicas are not tested to that standard. Gasket quality and case finishing differ. Do not dive with a replica. Service is the second problem. Most watchmakers turn away replica work, and parts for clone movements are hard to source through normal channels.
Price range
White gold Fifty Fathoms Automatique list prices sit above $20,000. Used examples trade below that. Replica versions sell between $300 and $700. Higher prices cover sapphire bezels, decorated movements, and thicker plating. None of those prices buy solid 18k white gold.