https://www.pricescope.com/diamond-prices/diamond-prices-cha...
That (crappy) $4.6k diamond would be from $3.3k on now :
https://www.bluenile.com/diamond-search/?CaratFrom=0.9&Carat...
So that's a -28%, and assuming no change between 2006 and 2008, this does seem to fit the recent drop in 0.5 to 1 carat prices seen on the chart ? (Note that all of the drops happened since early 2022.)
Meanwhile, the lab grown equivalent is around $1.6 k :
https://www.bluenile.com/diamond-search/?CaratFrom=0.95&Cara...
(Where did you get the $550 figure from ?)
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More interestingly, looking at diamonds that aren't crap (why would you settle for that when it's their optical properties that you are after ?), you have in the same size of 6.5 mm :
From $3.5k-3.9k for natural ones :
https://www.whiteflash.com/loose-diamonds/round-cut?carat=0....
(Clarity doesn't seem to matter that much, because it has not much effect as long as it can't be seen to the naked eye, which in these except the worst one it probably won't be ? https://beyond4cs.com/clarity/ )
From $1.3k for the lab-grown ones :
https://www.whiteflash.com/loose-diamonds/lab-created-diamon...
All the sellers I have seen so far but White Flash seem to at best have a tiny, monocolor «Hearts & Diamonds» logo - which makes it hard to judge for yourself. And at this point the prices are low enough that it doesn't seem to make sense to settle for less if your goal is something like an engagement ring.
I don't know, maybe I am exaggerating the importance of this - (and maybe moissanite or even cubic zirconium would actually make for even better optical properties) - but it does seem like that diamond buyers are overwhelmingly getting taken advantage of : https://beyond4cs.com/truth-about-gia-triple-excellent-diamo...
I was interested enough from this article that I was willing to try to find out by myself (including comparing with similar crystals) - but annoyingly White Flash is US based and doesn't export to my country.