What to Buy?

I can not tell you what to buy, but I personally buy the biggest, brightest, whitest, eye clean diamond (with some blue fluoro) for my wife; usually D-F, SI1. I know what to look for to see lower colour, but at 50+, I have no hope of seeing an SI1 inclusion!
 
If you are sharp eyed younger or "detail" oriented, you might prefer higher clarity, but smaller size. Higher colour and higher clarity combinations cost a lot more than high colour and medium clarity or medium colour and high clarity. Most people can see the colour difference between I and D, very few can see the difference between IF and SI1.
 
You may not mind seeing an inclusion or two, or you may find low colour diamonds "warmer". It is a good idea to view a range of diamonds to set your personal standards.

   

Carat weight costs the most, especially "Magic Weights" (like 1.00ct, 2ct, 1/2ct etc). "Undersizes" like 0.90ct and 0.45ct cost a lot less. If magic weights do not impress you, search for undersize; but they are rare, there are about 5% as many .99ct diamonds as 1.00ct stones.

A badly cut 1.00ct D Flawless costs more than twice as much a bigger diameter ( spread ), more sparkly Ideal Cut .90ct F SI1.

4 C's

Effect on

$

Beauty

Cut

$

Most

Color

$$

A lot

Clarity

$$$

Some

Carat

$$$$

?
 
50 years ago only very rich people could afford big diamonds. They often bought high clarity on the recommendation of a trusted jeweller (who stocked on the advice of his trusted supplier). Today we see more sales of F-H VS2 to SI1 diamonds.
 
A consumer is shown 2 diamonds - D IF and G SI. "They both look the same to me, but you want 3 times as much for this one? Are you kidding me?!"
 
Demand for high quality commercial grade stones (D-H, VS-SI) will probably continue to grow (in my opinion), and that will mean they will retain value and liquidity.
 
Religion or culture is an important factor; in some cultures clarity = purity = divine powers bringing good luck.
 
Shape . Round brilliants are the most sparkly and most popular shape; therefore rounds have the highest liquidity should you ever need to resell or trade up. Various fancy shapes come and go in popularity (and rise and fall in value). Right now, for engagement rings, Princess cuts are hot. Princess cuts have more small sparkles than round diamonds; but below 1ct, as we age, our ability to discern those tiny more frequent sparkles reduces. Princess cuts are more prone to chipping on the edges during wear and on the points during setting. To be fore-warned is to be fore-armed.
 
Investment Diamonds . Go see an investment advisor or buy property or shares. When you make enough money from your investments, buy a nice diamond!