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...