Can a jeweler tell if a diamond is lab-grown?

Not by eye.

Or even with a loupe.

Lab-grown diamonds ARE diamonds. They are chemically, physically, optically and thermally the same as diamonds from the ground.

If you use an ordinary LED tester, they register as DIAMOND.

However good jewelers will have a specialist tester that can identify MOST lab-growns. (There’s a particular type that can only be detected at a lab using VERY advanced equipment in the hands of a well-trained technician.) And if you’re trying to sell a larger diamond to a jeweler, odds are they’ll send it to a grading lab for professional evaluation.

Which is the point: lab-growns ARE the same as mined diamonds, but currently about 1/3 the price.

A few additional comments:

There is a lot of confusion between lab-growns, which ARE diamonds, and simulants, which are not. Simulants are things like cubic zirconium and Moissanite that simulate — or look like — diamonds but are not. A trained eye can detect these with ease.

lab grown diamonds come in different color and clarity grades, just like mined diamonds. I occasionally see comments saying that ALL lab-growns are flawless and colorless — and that simply isn’t true. Yet. What is true is that the AVERAGE lab-grown from a reputable producer is now of a higher color and clarity than the average diamond out of the ground. But you have a range of options, priced accordingly.