Tear of Offler
The Tear of Offler is the Disc's biggest diamond, uncountable carats of pricelessness. The gem itself was pried from the Statue of Offler, in the Lost Jewelled Temple of Offler somewhere deep in the jungles of Howondaland, Klatch, by Death, searching for a gift for a lady. The state of its friendliness is open to debate. The Lost Temple of Offler is referenced in The Compleat Discworld Atlas and is likely to be somewhere in the S'belinde region.
Annotation
The Roundworld reference is probably the Koh-i-Noor diamond, currently set in the Crown of the British Monarchy, dating from when Queen Victoria was proclaimed Empress of India in 1877.