Short answer
A fake Reine de Naples diamond is almost always a diamond simulant. Cubic zirconia, moissanite, and glass turn up in bezels and dials that copy Breguet's egg-shaped case. Stones are the easiest part of a watch to swap, so the reliable clues sit in the metal, the setting, and the watch itself: hallmarks, mount quality, dial finishing, and the movement behind the case back.
What shows up on fakes
Most counterfeit Reine de Naples pieces use round brilliant-cut simulants in a bead-set or grain-set row that mimics the genuine layout. Common problems include glue instead of metal beads, uneven spacing, stones that sit proud of the bezel, and settings made from gold plating over brass or steel. On the dial side, the fake usually replaces mother of pearl with printed plastic or a dyed shell disc, and the guilloche pattern is stamped rather than cut.
Prerequisites
- A 10x jeweler's loupe
- A multi-tester that reads thermal and electrical conductivity
- A long-wave UV lamp
- A gram scale accurate to 0.1 g
- Reference photos of the exact case number you are inspecting
- White paper and a bright, diffused light source
How to test the stones
- Clean the bezel with a soft cloth so skin oil does not mask the surface.
- Examine one stone at 10x and check that the facets meet at sharp, straight edges with no rounded corners.
- Inspect the setting from the side and confirm that metal beads, not adhesive, hold each stone.
- Look at the gaps between stones and note any uneven spacing or tilted stones.
- Place the probe of a multi-tester on a stone and read both the thermal and the electrical result.
- Repeat the test on three or four stones across the bezel rather than trusting a single reading.
- Hold the piece under UV light and note the color of any fluorescence.
- Breathe on the stones and watch how fast the fog clears.
A basic thermal tester alone is not enough. Cubic zirconia conducts heat in a way that can produce a diamond reading, which is why testing labs use instruments that also measure electrical conductivity. A stone that fogs and stays cloudy for several seconds is a strong sign of glass. Sharp facet edges, crisp light return, and a cool feel on the skin point toward diamond, but no field test replaces a lab.
How to check the watch
- Turn the watch over and read the case number and serial engraving with the loupe, then compare the fonts to reference photos.
- Measure the case across its widest point and compare the figure to the published specification.
- Inspect the crown at the 4 o'clock position and confirm the cabochon is a smooth polished stone, not a molded plastic dome.
- Study the dial surface at an angle and look for a cut guilloche pattern that shifts with the light.
- Check the hands under magnification for a clean, polished finish and correct Breguet shape.
- Weigh the complete watch and compare the figure to a known genuine example of the same reference.
- Look through the case back crystal at the movement and check for beveled bridges, polished screws, and engraved text.
- Inspect the strap or bracelet attachment points for a flush fit and matching hardware finish.
Red flags
- Glue visible around the stone girdles
- Stones that rattle or spin in their seats
- Bezel metal that shows copper tone at worn edges
- Missing or shallow hallmarks inside the case back
- A dial pattern that looks printed instead of cut
- Weight that feels light for the case size
- A crown cabochon that is warm to the touch and scratches with a fingernail
When to get a lab report
For any stone above roughly 0.25 carats, or for a watch you plan to insure, send the piece to a gemological laboratory. A trained grader can separate natural diamond from lab-grown diamond and from every simulant, and a written report carries weight if you later need to prove what you were sold. For smaller melee stones in a bezel, a bench jeweler with a multi-tester and a microscope can usually give you a firm answer in a few minutes.