I wondered if someones expert advice could help me to better understand something. 
As well as buying beautiful diamonds from David, in my journey to learn more I also purchased some very cheep diamonds on ebay mostly coming from Thailand.  They are in a variety of colors mostly with some sort of brownish accent color.  They are mostly round and under 20 points.  I was showing them to someone this weekend and we were sitting out side  - a beautiful sunny day around 4pm.
When we looked at the diamond with our body creating shade the diamond was a nice color and had a beautiful sparkle... but when held in the direct sunlight most of them looked a different color and some lost their sparkle.    I have looked at them outside at home other days and never really noticed this.  For comparison we held up the most recent purchase from David and even though the diamond was med blue flor it didn't change color at all and remained a beautiful yellow.... and boy did it sparkle.....

I don't have a UV light at home although i have just bought one online.   What do you think is the cause in this color shift.  Do these diamonds have  extremely blue fluorescence?  I had read with very strong flor some can appear cloudy...maybe that is what we were seeing.   Could they have been treated someway?  I bought a good diamond tester and they are real diamonds... The person i have been buying them from assures me there is NO treatment of any kind.  So what do you think is causing this?

I don't know anyone locally to take them to that knows anything about colored diamonds.  If you have any ideas of what might be going on here that would be great!


FD

Actually certain (not all) well cut colorless diamonds will appear gray in bright light because it is reflecting so much light that your eye makes the bright flashes look white and the remainder of the diamond look dark.  When we go outside our pupils will constrict more making everything look darker in general.  It's a shortcoming of our eyes/brains and not the diamonds.  My older RB cut does this in the sunlight and I actually don't like it as much. It has great fire though which I much prefer seeing over just bright flashes of light.

I doubt that a diamond that loses its sparkle in direct lighting is well cut though.  You will notice that the bright flashes seem brighter outdoors.  If you are losing this, I imagine the diamond is not well cut enough to reflect the light back in a positive manner.  

I really don't think that the fluor would cause this though.  From everything I have read it is very rare for an over blue diamond to look hazy.  Not that it is impossible, but more rare.  I only have blue fluor on my colorless diamonds and all the strong blues look fine in all lighting.  My poorly cut diamonds look very dull though.  ETA: My poorly cut diamonds are pre DBL

HI Everyone,
Complex issue.
Personally I find fluorescence can have a HUGE impact making a diamond look foggy in sunlight.
I agree that "overblue" stones are rare- however stones that are not "overblue" may look dull in sunlight.

I also agree with the points Squiggy made about how our eyes operate- but that alone does not explain why some stones look dull and others do not.

With regards to fancy colored diamonds looking dull in the sun:
There are combinations of brown body color with white fl that can make a diamond look completely opaque in sunlight- another cause is irradiation ( color treated diamonds) - or poor diamonds in general.
Imperfections, poor color ( that is to say color that makes the stone dull) and bad material can all play a part.

It depends... polish and symmetry you won't notice VG from EX without a loupe. GIA VG (and EX) cut proportions cover very very wide ranges, so your mileage really may vary.

It depends on why the VG is VG - for example a stone with truly excellent angles and crown height but a girdle that goes over slightly thick may be penalised with a VG. Yes, it may face up a little smaller, but it will outshine and outfire a mid-range "excellent" with a low crown and steepish pavilion. Also, my preference is for "old style" stones - high crowns, smallish tables, short lower girdle and broad pavilion mains. Most of them will end up as "VG" rather than "EX". Which is excellent for me, because they end up being slightly cheaper (though still difficult to find).

Irradiated stones come in all hues nowadays. The combination of irradiation and HPHT has made it possible to get much softer tones than the lurid greens and blues that were the mark of irradiated diamonds until recently.

For a "safe" answer, yes. Some treatments cannot be determined even with the most advanced lab analysis; this is what causes GIA to issue a colour origin of "indetermined" - it basically means "we have no belief that it is natural but cannot prove it is artificial". This is particularly the case with green diamonds (which turn green in nature due to irraidation)

I guess its just the natter of the direction of rays!!
Diamonds sparkle according to their cut... May be your diamond has better cut which helps reflect light and better fire and brilliance!! 

If you're looking for a natural diamond, no enhancement method is desirable.........