They are astronomically expensive, if you can even find a 6ct stone like you described.  David's got a .42ct Fancy Grayish Blue listing at $35k by way of an example.  And his stone is very well-priced! 

Put it like this - David had access to a 1.5 carat fancy deep blue, and that was about $1 million.

A 6 ct - if you could find it - would be way more than 4 times. Based on the price ratios of colourless diamonds, it could be about $10 up to 20-25 million.

BTW Allison - although the second one "looked blue", it was green. About as rare and expensive.

Me, too - blues are my ultimate favorite; but that light green is just killer.  

Thanks to OMC and David, I do have a little greyish-blue.  She was my hear's desire and is the center stone in my engagement ring, altho I wear it on my right hand.  My poor DH still just looks at it and shakes his head.  He can't understand how I'd prefer little and bloo to big and white.

When she's dirty she's very gray; but when she's been steam cleaned, she shoots blue sparks all over the place.

The main part is in testamonials - the Saga of Li'l Blooo.  http://coloreddiamon...saga-of-lil-blooo/0/  The actual find was in a thread by OMC, and has a link in the Saga.  Have fun with it.  I surely did.  It was almost 6 months of daily up-and-downs.  The crew here made everything so much fun, even tho I hadn't a clue what I was doing.