Until diamond prices crash, that is.
And they will - eventually. Aluminum used to be worth a fortune until people learned how to extract it easier. Diamond can by synthesized really well already, but prices are being kept artificially high. Diamond is way too good of a conductor to keep production low for long. I imagine that they'll keep trying to enforce ways to tell man-made diamonds from the others, but eventually that will fail.
I have been hearing that for a long time and my father used to hear it before me lol, diamonds are diamonds. I am sure the synthetic market {which has been around since the 1950's} will replace some of the "lower end' market but for the stones in the say $20K+ market, people are going to want "real" stones, insured, certified, and documented genuine diamonds made in the earth over millions of years, not a diamond made in a lab with a machine... All of the stones i have purchased are 100% doc'd and mark'd so they will at minimum hold their value...
heres a pic I took a while back, sitting on top of my tekton towers... thats less than half of her jewelry box, that is the white gold stuff, she has about half that amount in platinum {fad}, and about the same in yellow gold, 3 different jewelry boxes built into the safe, insured value is well over .75 mil, but actual would be more like 1/2's that, I want to say she is up to 125c in diamonds and 12oz gold.
For example-That ring in the left black box with the yellow stones is set in rhodium coated 18K with a matched pair of .75c genuine fancy vivid yellow diamonds and a 1.52 carat center stone certified GIA D/IF, the ring itself with no stones was only $1200{this is the second set of stones i had put in, the priginals went into a pendant}, the center stone was $40K {A STEAL AT THAT} and the pair of matched yellows were also a steal at $10K for the pair, I could sell the center very close to $60K and the sides for around 1/2 that, that is her second most valuable ring, the other one is all clear diamonds ttl over 5c!!!
here is a stone very similar to the center in her yellow tri stone...
http://www.ebay.com/itm/1-5-Carat-Round-Loose-Diamond-GIA-D-IF-Free-Ring-Cert-17157218-/400784848877?pt=Loose_Diamonds&hash=item5d50a377ed
SO they are investments, some day when I am gone and someone in my family needs funds down the line after they are handed down and inherited they will hopefully be worth millions....