The difference is instantly apparent under UV - most lab grown diamonds will not fluoresce unless they have a bad growth process that leaves flux and other impurities in the crystal.

Natural diamonds won't always fluoresce but the ones that do will do so in a variety of colors, and sometimes change depending on what wavelength is irradiating them.

Lab-grown diamonds can be tailored to exhibit the same impurities internal stresses, etc. that cause a minority of natural diamonds to fluoresce. This has not been a goal to date for synthetics because the highest price point is for diamonds that are most pure with least internal strain. If the economics of fluorescent diamonds suddenly become more attractive, I guarantee fluorescent synthetics will be on the market immediately thereafter, and will be indistinguishable from naturals without $100K worth of characterization tools.

Upside down and backwards.

The difference is not instantly apparent under UV.

Only about 30% of natural diamonds have fluorescence --- which is *caused* by impurities and imperfections in the material.

Manmade diamonds tend to lack this because they have fewer impurities and imperfections. Equating increased perfection and purity with inferiority is highly debatable and smacks of marketing BS.