If you were purchasing a diamond ring, how would you possibly know it was genuine, and not something industrial.
Shortly before my mother died she purchased a gold ring with three large diamonds. She purchased it at JC Penny about ten years ago. My father gave it to me, and since the ring had no sentimental value I decided to sell it. First, I took it to a jeweler and had it appraised. He appraised it at $500 - which seems ok, excpet Mom pait $1,000 for the ring! This is when I learned how careful you must be when purchasing a diamond.
There are fine diamonds, found in the best jewelry stores and 'trade' diamonds sold in large department stores and even discount chains. They are all real diamonds, not synthetic, but the quality is vastly different. The trade diamonds are sold to people who mostly want them for show, or I guess the word 'bling' can be applied here. Fine diamonds have a higher investment value. You're better off purchasing a smaller flawess, perfect diamond that a flashy trade diamond as far as investment value is concerned.
The safest place to purchase a diamond is at your neighborhood jewelry store, one that's been in busness for at least a few years. These stores depend upon word of mouth for increased business and will be there for you to maintain your diamond, such as cleaning and checking the setting to insure time or wear hasn't losened your stone.