Just to go play devil's advocate against my previous post and against some of the things being said in this thread: it's okay to spend a little on an engagement ring, so long as you budget for it and it's reasonable considering you and your spouse's income.
I get tired sometimes of people (on here and in general) bragging about how cheap their wedding or engagement ring is. "My wife's ring is made of solidified dust from the attic and cost us $4." Great, but you do you, and don't pass judgment on others.
I just recently purchased my GF's engagement ring in December and paid $2,100 for it. It's a beautiful, classic solitaire ring with a .5 carat diamond. The diamond is the top cut, top clarity, top everything. This is the ring here:
https://eshop.gabrielny.com/product/ER6672W4JJJI saved for that all year by putting $200/month in an account my GF had no idea about. I paid cash for it.
And I was okay with the expense because (a) we make about $115k combined so it was reasonable considering our joint income, and, more importantly, (b) we never, ever, ever do expensive things for each other. We've been dating for more than two years and have maybe gone out to eat 10 times. The most expensive thing I've bought for her is a Mickey Mouse/Pittsburgh Steelers blanket (two of her favorite things in one!) for like $30. The most expensive dinner we've had together was for my birthday, and that was like $115 up in Cleveland. We put a $50 cap on birthday and Christmas spending.
Given all that, I felt all our savings and discipline elsewhere allowed me to splurge a little on her ring.
So I guess what I'm trying to say is that you do you. If you want to treat your GF and it's within your means, go for it. An eight carat ring is still absolutely ridiculous, though.