The short answer

A fake Ballon Bleu large is a counterfeit copy of the Cartier Ballon Bleu de Cartier in its 42mm case size, the biggest of the round-cased Ballon Bleu line that does not have a chronograph. The genuine 42mm runs on a Cartier 049 automatic movement, which is built on an ETA 2892 base, and it sells for several thousand dollars on steel. Copies sell for anywhere from about thirty dollars at a street stall to a few hundred for the tier sellers call a first copy. None of them are Cartier, and in the United States both the sale and the purchase carry legal exposure under 18 U.S.C. 2320.

What "large" means in Ballon Bleu sizing

Cartier names this line by case diameter, not by a word like men's or ladies'. Large is 42mm. Mid is 36mm. There are also 33mm and 28mm versions, plus a 44mm chronograph that sits outside the normal large conversation. So if someone advertises a large, they should be talking about a 42mm case, about 13mm thick, with a smooth bezel, a sapphire crystal, and a crown tucked under a curved crown guard with a blue cabochon set into it.

The 42mm is the version most people picture when they picture a Ballon Bleu. It reads big on a wrist because the case is domed and the dial is large relative to the bezel. On a 6.5 inch wrist it takes up most of the top of the arm.

The tells I look at first

• The cabochon. Genuine is a blue synthetic spinel, faceted, set into the crown. Cheap copies use plastic or resin that goes cloudy, and the color skews toward royal blue instead of the deep navy Cartier uses.
• The crown guard. The fluted guard curves around the crown. On copies the flutes are shallow, uneven, or the guard sits too far off the case.
• The dial. Genuine has a fine guilloche texture with Roman numerals in a specific weight and a hidden signature in the VII. Copies often have flat printing, thicker numerals, or a texture that looks stamped rather than cut.
• The date. The window should sit at 3 o'clock with a crisp bevel. Replica date wheels use the wrong font and the numbers sit high or low in the aperture.
• The case back. Engraving depth, screw heads, and the reference number should match the case. Copies frequently reuse one back across sizes.
• The movement. A 2892 base winds smoothly and hacks. The rotor on a cheap copy is loud, and the winding feel is gritty.

Why the cheap tier fails

Most of the failure is in the finish, not the shape. Polishing on a copy is done by hand in a hurry, so the case edges go soft and the mirror finish on the bezel shows waves. The bracelet screws strip. Plating on gold-tone copies wears through at the clasp within months. The crystal usually has weak anti-reflective coating or none, so the dial looks grey under indoor light instead of black.

Legal and practical risk in the US

Trafficking in counterfeit goods is a federal crime, and US Customs and Border Protection seizes counterfeit watches in the mail on a routine basis. Purchases from overseas sellers can be stopped at the border, and there is no recourse when that happens. Beyond the law, there is no warranty, no service from Cartier, no parts availability, and no resale value. Nickel release from plated cases is also a real concern for anyone with sensitive skin.

What to do instead

If you want the look, buy a watch with its own design language that uses the same round case and domed crystal, or look at a pre-owned genuine piece from a seller with paperwork. If you want the real thing and the 42mm is too large, the 36mm mid-size wears closer to a dress watch and costs less used. Either path gets you something you can service, insure, and sell later.