Traditionally, one is supposed to spend two months' salary on a solitaire for an engagement.
"Solitaire" being, natch, a single stone. Typically a diamond (although some personal variation exists - I've also seen emeralds and sapphire solitaires given as engagement rings) and there is no typical guideline for the metal used for the setting. White gold, yellow gold, platinum, and Sterling are all acceptable (before getting a setting in yellow gold, you may want to make sure she's not allergic to the metal - it's rare, but it happens. My mother is allergic to all gold, and typically wears Sterling silver.) If you're going to surprise her, take your guideline from what she wears.
If you're going to get a solitaire, better cuts are "round" (just like it sounds,) "emerald" (a slightly elongated rectangle,) "princess" (very nearly square, lots of facets,) or an "oval" cut (you don't need me to explain this one, either.) Save the longer cuts (like a "marquis") for when you're buying a ring with her full knowledge, or if she wears a lot of that type of cut (a "marquis" cut is an elongated oval with mildly pointed ends...) because that's very much a personal preference. Try Googling <diamond shapes> if you want to know more.
Here's a sort of "how-to" article on buying an engagement ring -
http://honeymoons.about.com/cs/valentinesday/a/buyingadiamond.htm. Still, with all that it advises - the single best point I can give you is "go with your gut." Just because there's information out there doesn't mean you can let it make a decision for you - it just helps you make in
informed decision! Only you know what's going to be "right" - you know her best.
Your point about marriage and divorces is well taken as well - I've seen entirely too many divorces to want to rush into marriage. I ended up with a woman 17 years my senior - it's been ten years, and people still think we got married within the last six months. It was a struggle trying to find someone odd enough to put up with me for so long, but it has definitely been worth it! There was some friction (my mother is 18 months older than my wife, go figure...) but it's long done - the last holdout was mum, who has just gotten remarried and come around herself, I think.
Considering the amount you're "supposed" to spend, it's possible to finance the purchase - but even the amount is subject to change (I ended up spending more on a "five-year" ring - we have decided to exchange rings again every five years - than I did on the original engagement ring and all three bands (the original two and my own five-year ring - a titanium band.) Fortunately, she's not materialistic. I'd have trouble if I'd picked someone my own age, but who hadn't had any hardship to go through...)
If you want simple information on pretty much anything, good places to start are Wikipedia (
www.wikipedia.com) and About (
www.about.com.) I've used both rather extensively, and found them relatively free of aprocyrpha and useless rubbish...
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