Do not buy investment grade stones; if you get divorced you will regret it. - No the thing to settle on is a good quality stone
Remember firstly the 4 Cs, Carat (weight), Colour, Cut, Clarity.
Now on colour and cut, I leave that to you...personally I like emerald cut (rectangular ones) or step-cut aka square. Colour - well diamonds really do vary. They are graded from D to Z, D = totally brilliant to deeply coloured. A G to J usually works well
On Clarity, this is where a lot of your money can go. The grading is F (flawless) IF (internally flawless - .. some surface marks) VV1 and VV2 some tiny inclusions and small bubbles, then S1 and S2 (small inclusions and I-1 I-2 I-3 which are heavily included.
VVI and VV2 will give you good quality.
Now another carat = weight 1 carat = 0.2g and each carat has 100 points for subdivision.
If you are buying a 1, 1.5, 2 carat remember there is a huge price difference to going for a 0.99, 1.49, 1.99 - looks the same but can save you 20% plus.
Lastly use a platinum mount, they are stronger.
So now you are ready to go and ask for that 1.99 Em-Cut, VVI, G stone. Should be around £5-6k for that.....