The Cullinan Hotel is named after Sir Thomas Cullinan, the owner of the Pretoria mine where the largest gem-quality diamond was mined back in 1905. Before it was cut, the Cullinan diamond (also known also as The Great Star of Africa) had a mass of approximately 3106.75 carat - which is more or less 621.35 grams!
The Cullinan 1 diamond, cut from the raw Cullinan diamond, now has its home in Britain's crown jewels, forming the head of the Sceptre with the Cross.
There's an interesting story about how the diamond was transported to England via registered mail - read about hit in the Wikipedia Article.