mineral

Blue John Fluorite

Fluorite (Calcium Fluoride), CaF2

Hardness: 4 on Mohs scale; Color: Yellow, white, and distinct deep purple/blue banding; Luster: Vitreous (glassy); Crystal structure: Isometric (cubic); Cleavage: Perfect octahedral; Specific gravity: 3.1

Hardness: 4 on Mohs scale
Color: Yellow, white, and distinct deep purple/blue banding
Luster: Vitreous (glassy)

Identified April 4, 2026More mineral →

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Physical properties

Formation & geological history

Formed in hydrothermal veins within limestone. The unique 'Blue John' variety specifically formed roughly 270 million years ago during the Lower Carboniferous period through mineralized fluid deposits containing hydrocarbons and trace radioactive inclusions.

Uses & applications

Primarily used for high-end decorative ornaments, jewelry, and collectable display specimens. Historically used for making vases (tazze) and bowls.

Geological facts

Blue John is one of the rarest forms of fluorite in the world. Its name is thought to derive from the French 'bleu-jaune' (blue-yellow). It is only found in a single location: Treak Cliff Hill, Castleton, Derbyshire, UK.

Field identification & locations

Identify by its distinctive purple/blue and yellow color banding and its location of origin. In the field, look for vein deposits in limestone. Collectors should look for clear banding and lack of cracks.

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