After 1/4 century 'flogging trinkets' I think I can say why.
Most men get the idea they have to buy women something (that means little to them) that symbolisises their commintment to pledge themselves in perpetuity. (This is not natural for men, but to get what they want, these are the rules, so they play the game. Any one know the old bull / young bull joke?)

What does a sparkling bit of carbon mean to a sheila (aussie for girl)?
It will be compared with the symbol given to her peers by her peers bloke (aussie for guy).

Why am I bothering you with this drivel?
The best way I know to do well in this comparison (which centuries ago would have also been associated with the ability to support a larger family and still today - obvious social rank and standing) of survival of the fittest (Darwin - a place in Oz). Well what is it?

A diamond that out performs all others. And the solution - the Cut Advisor on this site.

You can find diamonds that out perform some of what have been previously defined as 'Ideal Cut' like the AGS 0's. And you do not have to pay the 15% to 40% premium that is currently command by these 'branded' goods.

This system is brand new - not even a week old (although I have been selling stones that meet these criteria for a decade).
I devised it primarily so I could buy from B2B sites in the early 90's - just like you are now - without being able to see the stones.

So imagine being at a dinner party, or coffee shop - sitting beside someone who has just bought a 1ct rock an hour ago - and it is pristine clean. Your .90ct measures a little wider across, and even though you have not cleaned it for a month - it just makes hers look like a cheap Cz or bit of glass!
You could loose freinds if you use this system, especially if you compare the prices you bought yours via Pricescope with what they paid!