diamond testers

I recently found a stone it has past many test, proven to be a diamond. It is a natural diamond, it is not cut. How do I go about getting info/ details on the natural stone, to see what it is worth? It is about 2.75 inches long by 2.5 inches thick by about 3 inches. How would I know what the carats are? Is there a special place to find all of this information?

Thank you,
Bonnie