I just bought my girlfriend a ring, though not an engagement. We've only been seeing each other for exactly 6 months come Aug 6th and her bday is next week, so I got her her favorite gemstone, white gold ring, with some smaller diamonds on the side of the gemstone.
First off, I recommend you go to K's Merchandice. They have a really large selection and a good price. I wouldn't recommend spending much on a diamond because of the whole situation with diamonds right now and how they are going to lose value. DeBeer's is controlling and holding onto most of the stock of diamonds, but once diamonds will be able to be mass produced to perfection at a decent cost, you'll see the price of diamonds plummet.