Oh jeez - this is such a dodgy topic because people can get very impassioned about it and whether it should be a diamond etc. And if you want a big ring then you're made to feel materialistic.
Here's my personal story (and no I don't want anyone to comment on it - it is neither right nor wrong...just how my story goes and I don;t give a hoot if you think I'm greedy. I'm a lady who knows what she wanted).
I have big hands. I wanted a solitaire - at least a carat. Anything smaller would look lost on my hand. I also wanted a diamond. It's a favourite stone, my birth stone and also classic - goes with anything (clothing wise) and is traditional (I'm sure that will be a hot topic - but I have read that the Roman's gave diamonds).
My fiancé is a keen mountain biker and has several - if he'll happily spend a few thousand on his bikes and their maintenance, I wanted to know that he'd put that much monetary respect*** (I'll explain what I mean by that in a minute) into something that I have to wear for the rest of my life.
I don't wear a lot of jewellery either, so wanted a wow piece.
the thing about diamonds and other stones is that the cheaper end of the scale means the quality drops. Less money means smaller, darker in colour, more inclusions in the body of the stone and not cut to a high standard. A badly cut, cheap diamond won't sparkle.
***monetary respect - knowing that if you want something quality you'll have to pay more for it. If you want a cheap skirt/coat/bag etc you can get them (do you have Primark in your country?)...but more often than not they will fall apart within a few wears. I don't even mean designer - I mean quality made. Same goes for rings - a ring isn't just a ring. They are all different and you really do get what you pay for. That said you can pick up a bargain
I also wanted something that I could pass down. An heirloom. There isn't much jewellery in my family and I wanted to be able to add to the 'prized family pieces'.
My fiancé went to the jewellery quarter in Birmingham (England) where he got a beautiful diamond valued at £2,000 more than he paid for it (smart shopping). I don't know where you are, but find out if you have a similar thing in your country. Basically its a huge collection of jewellers in one area, usually really close to a few goldsmiths and gemologists - hence the costs are kept down.
A couple I know bought vintage diamond and emerald rings (wedding and engagement) from a little jewellers for around £500-700...turns out they were worth closer to £5,000.
Thing is - this is between you and her and no matter how many people agree with her or you, it still is just to the two of you in this discussion at the end of the day.
She wants a different ring. You either compromise (but this is something I imagine is really important to her - for whatever reason and she may not budge...or be truly happy to) or you get her a different ring.
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