Is it impossible for a new player to discover a mine and start selling them for cheaper? Why doesn't this happen if they are so overpriced? They don't all go through the same company, so I would expect someone to try to undercut their competitors.
Mined diamonds can't ever compete with CVD diamond synthesis costs. As more CVD producers come online, I think we'll see prices for synthetics drop drastically. There's a lot of margin available for market-grabbing.
[1] http://www.bloomberg.com/news/articles/2016-07-05/first-find...
edited for typos
For now, the synthetic vendors are riding on diamond's traditional high pricing as sustained by artificial scarcity. They're cheaper than the naturals, but not all that much. I think it's their way of maintaining high margins during early business & market development.
I expect pricing to change when there are more suppliers and sales channels. Probably a couple of years to a definite downward inflection point.