As a former diamond buyer, I've been asked this question many times, worded in different ways:  "Which of the four-C's is least important when buying a diamond?"  My answer to this question?  Color.

When color-grading a diamond, the stone is held upside down on a white surface, to determine what degree of body color is visible.  This process makes color very obvious, much more so than it would ever be to the naked eye.  When the diamond is right-side-up, with the crown visible to the eye, color is much less apparent.  In addition, once placed against the skin (whether it be on a ring, pendant or earring) your skin color will be seen a bit through the stone, making any diamond appear slightly less than perfect. 

My advice to those in search of that perfect diamond is this:  If you need to save some cash, sacrifice color rather than size, cut or clarity.  You could save thousands, and the diamond will still be gorgeous.  That being said, J or K is as low a color grade as you should go, without seeing the stone in person.

Perhaps one of the most mysterious of the parameters on a diamond certificate is fluorescence.  Why does it matter?  Is it good or bad?  Allow me to shed some light…

Fluorescence simply lets you know if the diamond in question glows when placed under a fluorescent light.  Fluorescence in and of itself is not bad, and it can actually be an asset to a lesser stone.

Diamonds with little to no body color, (Like D, E, F, or G colored stones) may appear somewhat more milky to a discerning eye if they have fluorescence.  Diamonds with a darker body color (I, J or K) can actually appear less yellow if they have fluorescence.  Strong fluorescence is generally something to avoid, particularly when buying online.  Stones with a strong fluorescence can appear opaque, so you'd want to view this type of diamond in person before you purchase.  So you see, depending on the color of the stone, fluorescence can be a blessing or a curse! 

Trivia:  The hope diamond, perhaps one of the more famous diamonds in the world, fluoresces RED.  This unusual characteristic is one of many reasons people believe the owner of this gem would be perpetually cursed.  Do visit the Hope Diamond at the Smithsonian Natural History Museum if you're ever in Washington, DC!