you will know when you see it.
keep looking about.
I had my dh tearig his hair out over engagement rings, as I knew what I didn't want (same as you - no sticky out stones, can't abide them!), but didnt really know what I did want.
spent forever choosing - finally chose one.
woke up in a cold sweat that night as had chosen somehting I knew I wouldn't love forever.
back we went ot the shop...
couldn't choose (again!)
they lent me a ring (a Victorian half eternity) for a few weeks, so that I could get used to wearing one, and see what I liked/disliked etc.
went up the road for a coffee, adn thought "hmmm, this really is quite nice", and my engagement ring is a ring we designed based on that borrowed ring.
it takes time, but do keep looking. don't "settle" for something, as the lst thing you want is to get bored of it.