A replica Ballon Bleu extra large is a 42 mm copy of Cartier's XL Ballon Bleu automatic, and the copies worth owning get four things right: case thickness close to 13 mm, a domed crystal with the same curve as the original, a cabochon that reads deep blue instead of purple, and a dial with crisp, evenly spaced guilloché. Everything else is a negotiation. Get those four wrong and no amount of box-and-papers packaging will save the watch on a wrist.

What "extra large" actually means

Cartier sizes the Ballon Bleu by case diameter, not by model name alone. The extra large automatic sits at 42 mm, with a fluted crown capped by a blue synthetic spinel cabochon and a case that swells outward around it. There is also a 43 mm chronograph, so before you compare anything, confirm which one a seller is actually offering. Listings that say "XL" without a diameter are usually hiding something.

The four details that give a copy away

• Case thickness. The genuine 42 mm automatic comes in around 13 mm. Plenty of replicas sit at 14.5 to 15 mm because the maker dropped in a thick movement with a plastic spacer. That extra height makes the watch look flat and slab-sided, and the caseback stands off the wrist.
• The cabochon. It should be a saturated, slightly translucent blue with a smooth dome. Purple, milky, or visibly glued-in stones are the most common tell at the low end.
• Dial texture. The guilloché rings on a good copy are cut, not printed, and they catch light in bands. A phone macro shot at an angle shows you whether the pattern is stamped aluminum or actual texture.
• Hands and date. Sword hands need a visible center bevel. The date window should sit cleanly at 3 with a frame that matches the dial. Crooked fonts and off-center apertures show up immediately.

Movement choice changes the whole watch

Most decent 42 mm replicas run either a Miyota 9015 or an Asian 2824 clone. I lean toward the 9015 in this case size. It is thinner, which keeps the case proportions honest, and the rotor is quieter. The 2824 side usually wins on sweep smoothness and parts availability. Either is fine. What is not fine is a no-name movement with a decorative plate screwed over it, which is what most sub-$100 listings actually contain.

How 42 mm wears

The Ballon Bleu has long, curved lugs, so it wears larger than the number suggests. On a 6.5 inch wrist it takes up most of the top of the arm. Under that, the lugs start to overhang. If you are between sizes, the 36 mm or 40 mm version reads closer to a dress watch and avoids the dinner-plate effect.

Bracelet, strap, and finishing

Steel bracelets should have end links that sit flush against the case with no gaps or rattle, and screws rather than push pins at the links. Alligator straps are easy to fake badly: thin, glossy, and stiff means corrected leather. Check the clasp engraving depth too. Laser-etched text is shallow and gray. Proper stamping has edges.

What the price tiers actually buy

Under $100 you get mineral glass, a printed dial, and a movement that will drift. Around $150 to $300 you should expect sapphire, a 9015, and a dial you can photograph without embarrassment. Above that you are paying for case finishing, a better cabochon, and closer bracelet tolerances. Nothing in any tier is waterproof, whatever the caseback says. Do not swim in it.

Before you pay

Ask for a photo of the watch on a scale and a caliper reading of the case thickness. Ask for a macro of the cabochon in daylight. Confirm the diameter in writing. On the legal side, counterfeits are illegal to import into the United States and shipments get seized, and resale is not an option anywhere. Treat the purchase as a disposable cost, not an investment, and the whole exercise stays simple.