Hi,
As far as I know the biggest and most expensive diamond is the Culinan diamond. This diamond is set in queen Elizabeth's crown. I heard this fact while visiting in Culinan mines (some 80 k"m north to Pretoria, South Africa).
Besides I read a book about it. There was a farmer (Culinan ?) who lived in Orange free state (some 300 k"m south of Pretoria). One of his sheperds found a shinny stone. Send for analysis and found it is gold. The strory spread and hundreds of people entered the area each night. The owner sold the land for a huge sum of money and bought a huge farm in Natal (Area of Durben, far to the east side of South Africa). You won't believe it but the story repeated..... Gold....... He swore not to sell again.... but after so many troubles with intruders he gave up and sold..... Found a nice huge farm next to Warm-Bath (80 k"m north to Pretoria). One day one of his workers found........ not gold but diamonds....... And the biggest diamond ever found is that one.....
Ah..... some people have luck.......
Best regards,