When you're ready to buy an engagement ring make sure you checkout this site first.  Tons of great articles including one on Where to Buy An Engagement Ring which I think is fantastic.  They secret shopped dozens of jewlery stores and figured out the best places and the best options to buy a diamond engagement ring.

One good option:

Head to your local shopping mall.  You can find jewlers there in bulk!  And lots of engagement rigns to checkout in one swoop. Some major department storeseven carry diamonds (along with all those clothes, watches, and perfume counters). This allows you to see and pick from more diamond options in a short period of time. Don’t expect to have one mall retailer match another mall retailer’s price, though. They’ve coexisted in the same mall for years without slashing prices—they have no incentive to give you a better deal.

Or, of course, just google it.