First off, we're smarter than this. We see that DeBeers is just a corporation that has made buying a diamond engagement right a big thing through marketing. I agree, shower curtain ring should be fine if she loves you.
Unfortunately, not everyone is as slick, and the gf wants the diamond.
For the first engagement ring, get the biggest you can afford. Find out what shape of stone she wants, at a minimum.
Pick a price, then go to bluenile and find a stone in that price, at a relatively low quality - with a naked eye you can't tell if there are minor inclusions, etc. Also, if your gf doesn't know that diamonds shouldn't be really yellow, you can save money there too. (I was too stupid to do these things - I gave my wife a small, perfect stone.) Now take that info into 3 different local jewelers and see what price they can give you. Use the opportunity to look at as many stones as you can personally, with and without the stereoscope/microscope. It's a learning opportunity.
After 10 years of marriage, my wife made her engagement stone into a necklace, and I picked her out a new stone, about 4 times the price of the original, not a perfect stone. She picked out the setting, and everyone was happy.
BTW, I used student loans to by the first stone.
