I went for a moissanite/diamond compromise with my fiancee and it worked!
Charles and Colvard (the originators of lab grown moissanite) recently released a new generation known as Forever One and these cuts are currently the best on the market. They're brighter and have more sparkle than diamonds and are completely colourless. Essentially, they look better than any diamond money can buy and have the same properties in terms of hardness etc.
Despite all the advantages of moissanite (quality, price, ethics, environment etc), I knew a little part of her still wanted a real diamond on her finger (no matter how much she tried to pretend otherwise!).
My solution looked like this:
1. A 1 carat 6.5mm Forever One moissanite round centre stone (in a halo setting)
2. ~20 tiny, 0.3 carat real side diamonds (F colour, SI clarity - Note: these two metrics are much more important than the carat weight!)
3. 18K gold for the ring itself (bonus: the gold was recycled - feeble but worthwhile attempt to offset the environmental damage of the diamonds)
All told, it came in at under $1,500 (which, sadly, is ridiculously cheap these days for an engagement ring).
She loves it (and got her diamonds), we have more money to spend on other things and I'm happy I didn't lose too much cash and karma to the diamond cartels.
FYI, I did all of this online through https://www.moissaniteco.com. Highly recommend them!