Quick Answer: Each Diamond is unique, you can't find two diamonds exactly alike. This is why you have to see and choose your diamonds side by side with your own eyes . . . Period.
The best example is a recent search with an on-line diamond seller; they had more than 14 Round Diamonds that were 1 carat in weight, G in Color, SI1 in Clarity, Ideal Cut and all with GIA Lab Reports. The Highest price was $7,900 and the Lowest was $4,800. Why is there such a price range for such similar diamonds by paper and on the same site ? Again the answer is that each diamond is unique, you can't find two diamonds exactly alike. Simply, Lab Reports can't tell the unique personality that each diamond has. Then, each individual person has different likes and dislikes when looking at diamonds, even among trained Gemologists.
This is why you have to see and choose your diamonds side by side with your own eyes . . . Period. This rules out almost all of the on-line avenues, as most don't or can't allow you to do a true side by side comparison.
You are not buying a piece of paper you are buying a Diamond, so look at Diamonds.
You are not going to propose with a piece of paper you are proposing with a Diamond, so again look at Diamonds