This assumes you are going the diamond route. The points below about the relative value, stone suitability, and marketing around diamonds vs. other gems are all true. That having been said, the DeBeers machine is formidable. Sometimes it's best to stick with the most common route.
1) Hie thee to pricescope.com. They will arm you with all the information you need to make an informed diamond purchase, online or local.
2) Cut matters most. A well cut diamond will look 100% better, and also may measure up to 20% larger in diameter, than a poorly cut higher-carat diamond. "It's so sparkly" is a good review from any diamond recipient.
3) Three months salary used to be two months salary, and depending on your salary, may be too much or too little anyway. Pick a purchase you are comfortable with. It is one of the few times that you are buying something for life, though, so spend what you can within reason. Odds are you will not regret it.
4) Spend some thought on the proposal. That is a large part of the memory you both will share that will keep you connected in trying times down the line.
Good luck. Don't let your single friends snow you, a good marriage is the single most valuable thing in a man's life if he is lucky enough to find one.