I found this story on Futility Closet's site here.
"The largest gem-quality diamond ever found was discovered in a South African mine in 1905. The so-called Cullinan diamond weighed 3,106 carats, or about 1-1/3 pounds."
A replica of the uncut stone.
"The Transvaal government purchased the stone and offered it to Edward VII on his 66th birthday, but the problem remained how to transport such a valuable object safely to England. Amid great publicity and heavy security, an ocean liner set out carefully for London.
"It was carrying a decoy stone — the real diamond was sent by parcel post. It arrived safely."
If ever a mailman deserved a tip . . .
This diamond was shortly thereafter cut into several smaller, wearable diamonds (the weight of the original being far too heavy to put it in any sort of setting). It was split into these large nine pieces (the two largest being named "The Great Star of Africa" and "The Second Star of Africa"). These are glass reproductions of the finished stones:
Apparently the technology to split such a stone was not very sophisticated at that time, so the jewelers selected by Edward VII (the Asscher brothers) were very nervous that they would ruin the entire thing. They basically had to cut an incision deep into the stone and then, just as if they were splitting rails, stick a wedge in and wallop it with a hammer.
"The tale is told of Joseph Asscher . . . that when he prepared to cleave the largest diamond ever known, the 3,106 carats (621 g) Cullinan, he had a doctor and nurse standing by and when he finally struck the diamond and it broke perfectly in two, he fainted dead away". Yeah, I'd be nervous, too, if I was taking a sledgehammer to something worth over £1 billion.
These gems are all now in the crown jewels of Great Britain.
Here's Cullinan 1 in the Imperial Sceptre:
Here's Cullinan 2 in the Imperial State Crown (just underneath the giant ruby):
Here's Her Maj wearing Cullinan 3 and 4, clipped together in a brooch:
Here's Her Maj wearing Cullinan 5:
Here's Queen Mary wearing Cullinan 6, 7 and 8 (6 is the diamond amidst the emeralds on her "stomacher" pinned to the bosom of her dress; 7 is the diamond hanging off the chain of her necklace next to the emerald on the other chain; 8 is the diamond dangling from the bottom of her stomacher, right above the final dangling emerald.):
Finally, Cullinan 9: