Hi,
Just thought I gave my 2 cents. For me out of the 4Cs, the one thing you should never ever sacrifice is the cut. The cut is everything. Go nothing lower than a triple excellent. Always get a certified diamond. GIA is the industry recognized standard. Unless you are going for a Tiffany, Cartier, Lazarre or something of a premium branded diamond, settle for nothing less than a GIA certified diamond. Some diamonds are classified as IDEAL cuts by other labs like AGS, IGI, EGL, etc.. but these labs are known to be "looser" on QC. This would mean that if a diamond is certified as IDEAL cut or say a colour D or a clarity of VSI1 by other labs, they may not fair that well if it is examined by GIA. It is a well know thing that diamonds that fails excellent grading by GIA are sent to other labs to get an "upgrade" on the grading.
Now why should one never compromise on cut you may ask right? Because say a 100pts poorly cut diamond vs a 80pts excellent cut diamond. You may realize that it looks the same. The reason why is because of the proportions. Meaning the the diamond may be "heavier" at certain areas or "longer". If I could put it in layman terms, say "Woman" A and "Woman" B both weights 50kgs. Person A may have the the vital stats of 34-24-34... and person B may be 20-33-34. The same applies. The cut is VERY important.
Next you should lookout for is the colour. D,E,F are colourless colour and this range of diamonds are in the premium colour range. Now again I will come back to the importantance of a triple excellent diamond. The sparkle or fire and brilliance in a diamond between a triple excellent diamond of say colour G may shine and sparkle better than a E coloured poorly cut diamond. So though the sales person may push you to buy a D or E colour saying it is crystal clear and such, reality is the diamond will not sparkle. It will look dead and dull. This is because a well cut diamond will retain the light that goes in and hence you will be able to see very clearly the fire and brilliance in it. A poor cut diamond will deflect light out of it at the wrong angles. Having said this, if you are on a budget and are not looking at being a collector, I think F, G or H is of great value for an engagement ring. However, make sure the cut is triple excellent and nothing less. Btw, if the sales person try to sell you a diamond by showing you both a D and a G diamond from the "top" which we normally call table of the diamond, and tell you they are of the same colour, they are trying to cheat your money. From the top, only a real well trained expert can tell the difference. We can't, tilt the diamond upside down and view it from the pavillion. Put both diamonds side by side and you will see visible difference. Go nothing less than a H. You do not need a D or E because they are normally for collectors and the price jump between a G and E for example can be huge.
Next thing you should look at in my opinion is Carat. This is the most visible one. We can tell how big a diamond is by its carat weight. However again, the cut must be nothing less than excellent.
Clarity is the last thing I think you should look at. Forget the anything higher than the VVS range. This are premium diamonds for collectors and the price jump is huge. You cannot see clarify with the naked eye. The clarify of the diamond can only be seens under at least 10x magnification. If you go to a store, ask them for a loupe and you see the "dirt" in the diamond. I would say for an engagement ring, get something like a VS range or SI is fine. Some SI can be quite "dirty". However this cannot be seen with the naked eye. I would say go nothing less than a SI1.
Conclusion, a well cut diamond can look better and bigger than a poorly cut diamond of a higher carat. So settle for nothing less than a GIA certified triple excellent diamond. Then play around with the rest of the Cs until you find one that suits your budget. On stores which carries good diamonds, go to degem's HQ in bangsar, or Suen. They are pretty professional and can educate you very well without being pushy. Some stores just want to push you to make a sale. Don't believe me, walk into Tiffany or Cartier let them educate you. You don't have to buy anything but if you walk into one of these (Degem, SUEN, Tiffany, Cartier) and then make a stop to the others, you will notice the difference. Many will try to just sell you a bad diamond with a higher carat. Some will convince you that certification is not important. Some will even push you to look at other certifications other than GIA and tell you their certs are better or the same as GIA. Don't believe them. Lastly, happy hunting.