Blue John is a very rare, semi-precious mineral with bands of purple-blue or yellow colour. The mineral is a highly prized and extremely unusual one, as Blue John can only be found in two locations both of which are in Derbyshire in the UK. In the 19th Century, Blue John was mined extensively due to to its beautiful appearance and was used to create magnificent pieces of antique jewellery. Today, the mining of Blue John continues, but on a much reduced scale.