Visually, what's the difference between a natural and lab-grown diamond?

From two feet away? Nothing. Not a thing. I've set thousands of stones and I can't tell them apart with my eyes alone. No jeweler can, not reliably. The carbon crystal structure is identical, the refractive index is identical, the dispersion - that fire that comes off a well-cut diamond - is identical. You're looking at a diamond, period.

The differences live under a microscope, and even then, they're clues, not guarantees. Natural diamonds often carry tiny included crystals - a speck of garnet, a bit of diopside - that tell a story about the mantle where they grew. Lab-grown stones, especially HPHT, sometimes have metallic flux inclusions: little black specks of nickel or iron that look different from anything the earth makes. CVD stones, which are most of what's on the market now, can show faint growth striations - parallel lines you'll see if you know to look for them. Most clients won't. Most jewelers won't bother.

There's a color nuance some people claim they see. CVD lab stones occasionally have a subtle grayish or brownish undertone before post-growth treatment, and HPHT stones sometimes carry a faint blue cast from boron. In real life, under jewelry store lights? Forget it. I once had a client, Priya, bring in a 1.4 carat oval she'd inherited, convinced it was lab-grown because it was "too white." It was a G-color natural with strong blue fluorescence. Looked like a lab to her. The GIA report said otherwise. That's the only thing that says otherwise.

So if you're worried your lab diamond will look obviously synthetic to other people, don't be. It won't. It's a real diamond. If you're buying a natural stone and want to verify, you're not going to verify it with your eyes. You'll verify it with a grading report from a lab that actually checks for these things. Without that paper, it's just a shiny rock and a story.