As a chick, I feel obligated to put in my
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However, my brothers, you've covered all the bases marvelously which leaves me very little to add other than my personal opinion and a brief anecdote.
There's a reason women refer to them as "sparklies". When talking about rings, we talk about settings, styles, and metals, because it's assumed: whatever size/shape the stone is it will be high quality. I've never said to myself, I'll compromise on quality for a bigger stone. I've only heard two women express disappoinment about their engagement rings. One was disappointed in the size of the center stone. The other felt her ring was missing what she called the "kachow!" factor. The first managed to get 4 size "upgrades" on her engagment ring (trading in the "old" one for a bigger one, and yes I mean 4 seperate transactions) and was divorced before they celebrated their 1st anniversary. The second had a very uncomfortable conversation about the situation with her fiance after which they went shopping for a different ring. Now, it's a smaller stone but much more brilliant... and the "kachow!" factor is definitely there.
With all this wonderful advice on what to look for, I'm certain you'll do fine.