Short answer
A fake Fifty Fathoms sold as white gold is almost never gold. Most units use a 316L steel or brass case with rhodium plating 0.5 to 1 micron thick. Some use a tungsten shell to fake the weight. Blancpain has sold Fifty Fathoms references in 18k white gold. Reference numbers change by year, so treat any single number as unverified until you see the case.
What 18k white gold is
18k gold is 750 parts gold per 1000. White gold adds palladium, silver, or manganese to bleach the yellow tone. A rhodium layer 1 to 3 microns thick covers the finished case. Density for 18k white gold sits near 15.5 to 16.5 g/cm3. Steel sits at 7.9 g/cm3. Pure gold sits at 19.32 g/cm3.
Weight test
Same case volume, two times the mass. A 45 mm Fifty Fathoms head in steel comes in near 100 g. The same case in 18k white gold comes in near 190 to 210 g. Tungsten has a density of 19.25 g/cm3, so a tungsten fake can match that number. Weight alone proves nothing. Weigh the watch, then test the metal.
Build types in the replica market
- Plated steel: 316L case, rhodium flash 0.5 to 1 micron. Weight near steel.
- Brass with plating: heavy for its size, plating wears through at crown guards and lugs.
- Tungsten shell: correct weight, wrong metal, and it rings under a scratch test.
- Solid 18k aftermarket case: rare. Needs an assay hallmark and a fineness stamp.
Checks that separate gold from plating
- Hallmark. Swiss assay marks sit inside the caseback. 18k gold carries the 750 stamp plus the assay office mark.
- XRF reading. A handheld XRF unit reads the top 5 to 20 microns. On plated steel it returns rhodium and iron, not gold.
- Specific gravity. Weigh dry, weigh suspended in water, divide. A plated steel case returns 7.9 to 8.1.
- Wear points. Plating thins first at the crown, the lugs, and the caseback edge. Look for a yellow or grey line there.
- Magnetism. 18k white gold is not magnetic. Ferritic steel parts pull a magnet.
Movement and finish
Replicas run Asian clones. Common bases are the Miyota 9015 at 42 hours of reserve and the Seagull ST21. Genuine Calibre 1315 uses three barrels, 227 parts, 35 jewels, and a 120 hour reserve. Check the balance bridge and the rotor engraving. Replica rotors show shallow laser etching, uneven lettering, and a decorated plate set over the movement.
Size and thickness
Fifty Fathoms Automatique specs: 45 mm across, 15.4 mm thick, 300 m water resistance, sapphire bezel insert. Many replicas measure 44 mm by 16.5 mm, or 45 mm by 17 mm. A caliper on the case flank settles it. Replicas also fail pressure testing at 300 m. Most hold 30 m to 50 m.
Prices
Replica Fifty Fathoms units sell from about $300 to $900. A plated steel version sits at the low end. A tungsten or gold plated build sits at the top. Genuine 18k white gold Fifty Fathoms references trade from about $18,000 to $30,000 on the secondary market. The melt value of a 100 g 18k case runs into four figures at any recent gold price. A $700 watch with a solid gold case does not exist.
What I cannot confirm
Blancpain does not publish a full list of white gold Fifty Fathoms references. Reference numbers and production years change by market. Treat any seller claim about a specific white gold reference as unverified until you see the assay hallmark and an XRF reading.