I got married last year and bought the engagement ring about 2 years back. I think diamond prices are roughly still the same. Ok first you got 2 types of rings you need to buy. To get her to marry you, you need to buy an engagement ring. These are usually diamond rings and could be a solitaire or all kinds of fancy designs. After she agrees, you need to buy a pair of wedding rings. Usually this are simple bands of white gold/platinum/gold. Diamonds optional.
Next you have the 3 tiers of diamond rings. Highest level are Tiffany & Co/Cartier etc level rings. These are expensive, branded and highly marked up. You are basically paying for the brand and the "box" that it comes in. Your wedding photographers will definitely be taking photos of rings in the box so if that's your priority for face, then its up to you. Personally you are paying 3-4 times market value just for the brand and I think it's silly. Good Half carat (triple excellent, VS1 or better, F Colour or better) will cost you 20k an above.
Medium tier is the pseudo luxury brands such as Lumiere (by Tomei), Habib (Hearts on Fire series) etc etc. These are branded and cost 50%-100% above market price. However you do get slightly better diamonds. Make sure you get "hearts and arrows" diamonds. These are premium cut diamonds that are apparently more shiny "due to how well they are cut @ the ideal angles". If you examine with a hand loupe, you can see a hearts and arrows pattern in the diamond. I went for this level of diamonds lar, no Chinese Jewelry shop brand name and not as expensive as true luxury brands. Also you pay for the cutting quality as well. Good Half carat about 10k.
Mass market level are Poh Kong, Wah Chan, Tomei etc etc level. These are the best bargains for your money. Make sure you are buying GIA diamonds (diamonds inspected by Geminological Institute of America). Nothing much else to say except these are the best value diamonds. Good Half carat about 6-7k.
Next is the 4cs. Carat, Cutting, Colour & Clarity. Carat is weight (and thus size). Cutting is how well it is cut (how shiny it is). Colour is how clear it is (D is perfectly colourless, I/J/K above, diamond gets abit yellowish). Clarity is how perfect the diamond is without inclusions (VVS is very very slightly included, SI1 and SI2 is slightly included etc).
My 2 cents, go for the biggest cheapest rock you can find. Get a diamond that is as big as possible with crappy colour and clarity because honestly nobody can tell without a microscope. Cutting is pretty important to make sure that the diamond is sufficiently shiny. Just as a general quide diamond prices triple everytime carat size doubles. so if half carat is 7 k for a Poh Kong level ring, it'll be 20k for a one carat and 60k for 2 carats.
My advice is buy slightly below a round number. Buy 0.48 or 0.92 or 0.97 etc. The pricing is cheaper and it'll look equally big. For 3-5k, don't bother with the luxury names. Just survey Wah Chan and Poh Kong for GIA diamond rings. You should be able to get a good quality 0.3++ or a crappy quality 0.4. I suggest go for size.
Forget about resale value for diamonds. The buyback programme I was offered was 25% of purchase price and this is only for trading up. Better buy a cheap diamond ring and buy a nice watch. Also weddings are EXTREMELY EXPENSIVE. Please budget for wedding photographers and wedding entertainment in addition to your wedding dinner. Wedding photographers/videographers for prewedding pictures and actual day photographers/videographers cost me almost 15k. Wedding live bands + emcee will also set you back about 5k.