Somehow i doubt that would be a good investment... i think diamons may lose quite a bit of their value with synthetics ever improving. Its mostly a matter of supply and demand in these things, even for gem applications.
I have no doubt at some point the synthetic diamonds will become gem size, colorless, and possibly with clarity rarely seen in nature (FL/IF). As it is, 'being natural' isnt all that important with diamonds - would you consider a radiation treated/improved diamond to be a natural stone?
Value of any gemstone is set by a couple primary conditions other then clarity, cut, etc.:
1: Market Supply, vs Market demand
2. Confidence in the buyer of the genuine nature of the stone.
An example: The mineral Corundum (read sapphire and ruby) has been synthesized for over a century. (right around 1900 or so, I can't remember the exact year to be honest I am too lazy atm to look it up) Blue sapphire of course, you simply dope it with titanium and iron, vs ruby you dope it with chromium. For a star sapphire you dope it with rutile. (titanium dioxide). The processes of which have been long since perfected, any optical grade of corundum can be produced with ease now. This has of course been of critical importance to the early production of lasers, as a ruby crystal laser from a natural crystal wouldn't be happening anytime soon. The latest advancements in the synthetic field have been based around synthetic color enhancement in natural sapphire. (read: Diffusion treatment)
The wide availability of lab grown sapphire and ruby hasn't had as much of an impact as other conditions have. You see, there are ALWAYS people willing to pay more for a "natural" stone. And you can easily identify a lab stone, so the market supply and market demand for them hasn't changed much from the addition of lab stones in the past century. Enhanced mining processes have on the other hand, GREATLY increased the market supply, rapidly outpacing demand. This has hurt the price severely.
Diffusion treatment for color enhancement has had a MUCH bigger impact. In diffusion treatment you add chromium powder, or titanium dioxide, or iron, or titanium, etc. Pack it around the stones, then heat to a fairly high temp for a prolonged period of time. the powder diffuses into the surface, enhancing the color to the desired color. The problem with this process is it leaves flaws inside the stone intact. So less savory sellers have been known to sell diffusion treated stones as only "heat treated." Heat treating corundum has been around since the Roman Empire was still flourishing, and is widely accepted.
Diffusion treatment is not as easily determined, so plenty of stones are on the market today as natural. This hurts buyer confidence of the genuine nature of the stone, and it's the destruction of this confidence that hurts the market demand, lowering the price via simply supply/demand economics.
You can tell easily if a sapphire has been heat treated, simply look for internal crystals of another material that have "exploded" inside. (among other signs) however, heat treating is simply diffusion treatment of the natural inclusions inside the stone, enhancing the color with existing elements inside the stone. since nothing is added in standard heat treating, it's generally considered fine for a stone to be heat treated, so upwards of 95% of the corundum produced annually in this world is heat treated. It's the problem of another method, that can produce ANY desired color, that closely mimics this process, and is quite annoying to identify, and can be HEAVILY mass produced, that produces the significant damage to the price we see today. (that and trade embargoes against source countries like Myanmar (read: Burma)
Diamond on the other hand has no diffusion treatment. Irradiation can be spotted to determine likelihood of natural color, and the crystal structure of a lab grown can be identified relatively easily. Also unlike corundum, diamond crystals are actually quite difficult to mass produce in gem grade, this limits the impact on the price. Add in a fixed market, courtesy of de beers, and you have a recipe for a stone that won't be changing in value ANYTIME soon.
If you want a good investment stone, however diamond won't be increasing either, again thanks to the fixed market, Instead your best bets are tourmaline and emerald. Given the larger deposits are starting to run out. Granted there are other deposits, and with tourmaline, they are on either side of the atlantic, as the deposit is older then the atlantic ocean, but the African deposits are much smaller then the Brazil deposits. Also, there are theoretical other large deposits of emerald comparable to Columbia. Utah is home to one of the suspected emerald deposits, however they have never been found. (however utah has plenty of other colors of Beryl, such as aquamarine, and even red, which is unique to utah, probably why they expect the green beryls to be here, albeit on the other side of the state from the red and blue.) Ok I am babbling here. I should probably have ended this an hour ago...
StridAst
(Rockhounding is what got my interested in jewelry and led me to my career.) I actually can't wait until next year when I head down to topaz mountain again. There is supposed to be a lot of rather nondescript chalcedony there that fluoresces extremely bright due to uranium in the deposits there. Already checked my topaz and red beryl, none of it fluoresces with my PHR, Some of my opals fluoresce brightly, and I have a lot of color change sapphires that glow as red as my ruby collection does when I hit them with the beam. (the color change ones go from pink to violet) Actually any color change gemstone will fluoresce, as that is what is causing the color change. so all alexandrite will as well. Also moissonites fluoresce nicely too.
