No matter where you shop, here are three things to look for to make sure you get what you pay for.
Look for a certified stone.
Buy a diamond that comes with a certificate from the Gemological Institute of America or the American Gem Society. That way, you know exactly what you're getting.
Find out the refund policy.
Make sure the store has a written cash refund policy. Both Tiffany and Costco do.
Get the diamond appraised.
Immediately after you purchase the diamond, take it to a qualified diamond appraiser.
You can also test diamonds yourself using a device that operates on AAA batteries.
Consumer correspondent Elisabeth Leamy used a Presidium Multi Tester that retails for about $250 to test diamonds on "Good Morning America."
The device measures how quickly the gem stone tested cools the heated tips. Because diamonds are the best heat conductors in the world, they cool the tip of the device almost instantly. Other materials like moissanite -- a man-made gem stone with many properties similar to diamonds -- will cool the tip more slowly.
ABC News consumer correspondent Elisabeth Leamy originally reported this story for "Good Morning America Weekend Edition."