Um, I don't know, Dresden - synthetic diamonds have been made repeatably since the 1950s, and are commercially available to the public since at least the late 1990s (maybe even earlier). Differently to what happened to pearls and corundum (but curiously not spinel or alexandrite), the availability of synthetics seems to have left prices of natural diamonds pretty much unaffected, at least so far. They have gone up, come down, gone up again, but all in blissful ignorance of the synthetic market.
On the "greenness" of synthetics, I am also relatively doubtful - it does take a huge amount of energy to synthesise a diamond, though it is perhaps a valid argument for coloured diamonds (especially blues, that can be grown with pretty much the same energy expenditure as colourless, but are much rarer naturally, requiring significantly more excavation work to find). This said, I have never seen an "energy balance" for either natural or synthetics, and it is very possible that synthetics hold the energetic upper hand, especially with the newer production methods (CVD) and considering exhaustion of natural sources.
Finally, AFAIK only colourless, yellow and blue can be synthesised "fully naturally". All the other colours require post-synthesis treatment like irradiation or HPHT. Size is also a problem, though there seem to be claim after claim of large (20+ ct) products, commercially available samples are rarely over 1.50 ct once cut. Perhaps the energy equations tilts the other way for very long production times?
The really interesting part about synthetic diamonds, for me, is the ability to produce monocrystalline sheets that can be used as a replacement for silicon or other materials (Ge, GaAs) as semiconductors in electronics. Unfortunately, I think we are still a couple of decades away from that...