> His lab can tell the difference — they use microscopes and other instruments to look for subtle features that reveal a diamond's origin. Wild guess as to why anyone can tell the difference: Lab-grown diamonds are more perfect than mined ones. It's probably pretty trivial to introduce impurities to degrade the color, or change the deposition temperature in order to create faults in the lattice. Hell, throw in a micr…
As a producer, I don't see any incentive to make them imperfect on purpose, yet still distinguishable from mined diamonds. All larger diamonds intended for gemstones come with independent grading reports, which will still identify it as grown regardless of presence or lack of impurities and imperfections.
While not technically correct, some of the advanced detection equipment can be thought of like looking at growth rings on a tree. You (probably) can't replicate the growth rings of a 200 year old tree in a 5 year old tree, yet both are perfectly valid sources of wood (sustainability, etc. aside).