Are there any perceptible differences between these and 'natural' diamonds?
Source: After reading this Wired article [1] ~15 years ago, myself and two friends tried to (unsuccessfully) put VC money together to commit to large amounts of manufactured diamonds for the consumer industry.
I still have a small block of manufactured diamond I had made with impurities (as a proof of concept) in a box in my closet. With age comes wisdom.
[1] https://books.google.com/books?id=2GrqeGCE544C&pg=PT142&dq=I...
> In 1891, B. Walter was the first person to observe the well known cape spectrum in natural diamonds. This series of bands associated with type Ia natural diamonds (that are usually colorless and have blue fluorescence to ultraviolet light), are usually visible in the blue to violet part of the spectrum. ... No synthetic diamonds will show a cape spectrum.
It goes on to describe other types of visible tests.
The next capture (which I can't fully see on Google Books) describes a device called DiamondSure that fits on a desktop and looks at the cape fluorescence and absorption of particular wavelengths.
Synthetic diamonds can be readily detected by at least two instruments sold by De Beers.[1] They look at emissive and absorptive optical features that aren't due to impurities, but due to defects (vacancies, stacking faults, interstitials, etc.). These crystal defects are atomic-scale departures from lattice perfection that are inherent to high growth rates (compared to natural diamonds). You can't see them on visual inspection.
These machines are expensive, even when sold at De Beer's cost. You won't see them in the corner jewelry store. But for stones > a carat, it's worthwhile to send a gem to services that use these machines to determine whether the stone is, for example, a natural type IIa (the purest of natural diamonds) or a potential synthetic that "needs further characterization".