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Bluejohn
From Discworld & Pratchett Wiki
First encountered in The Fifth Elephant he is the largest troll ever to be in the Ankh-Morpork City Watch, Bluejohn is a very effective riot shield. Currently, as of Unseen Academicals, an officer of unknown rank.
[edit] Geology
He is named after Blue John, a bright blue mineral once mined in Derbyshire, England.