You’re standing outside Treak Cliff Cavern, one of only two places where Blue John is mined. (the other is Blue John Cavern, also nearby). Blue John is a variety of a mineral called fluorspar, composed of calcium fluoride (CaF2). Fluorspar comes in a range of colours, but Blue John is characterised by bluish purple with white or yellow banding. Its colour and translucency have made it popular for creating ornaments or jewellery – but these qualities have also been exploited to create decorative ‘windows’, akin to stained glass.
The window above was made by Castleton-born John Tym. It is made in three pieces, each containing thin slices of Blue John glued onto glass. It was commissioned in the late 1800s for Birchfield Lodge, Hope, where it remained until 1983. It is now reframed in a light box and on display at Buxton Museum and Art Gallery.