Blue John is a rare mineral, primarily found in the caves of the Peak District in England. It is a type of fluorite, known for its striking blue and purple colors, which can vary in intensity. The mineral is often used in jewelry and decorative items due to its unique appearance. The name "Blue John" is believed to have originated from the French term "bleu jaune," meaning "blue yellow." This mineral has been mined since the 18th century, and its distinct hues make it a sought-after material for artisans and collectors alike.