Bear in mind there is a budget for your ring. Shop around to local jewelry stores you can trust. Be sure you have a written appraisal of your rings accompany your purchase. Don't insist your fiance' makes a purchase on an Engagement that is out of your budget. Keep your budget in mind and stick to it.
If you are "surprised" with an Engagement ring and did not get to pick it out, please accept it graciously! The world isn't going to end. There will be Anniversaries and milestones to share later where you will either be able to afford your dream wedding set and it will be meaningful.