How much is a 1 carat d flawless diamond worth?

Depending on the cut and exact carat weight, you can purchase such a diamond today from Blue Nile and spend anywhere between US$9,500 and US$13,100.

If you already own such a stone, best practices dictate that you pay for a certificate from a certified gemologist to document this stone. Then as you try to sell it, you have the proof you need that this is indeed a valuable diamond.

Finally, anything is only 'worth' what someone will pay you for it.