Blue John is a rare, banded variety of fluorite characterized by its distinct, often dark purple-blue and white concentric banding. It is found almost exclusively in the Castleton area of Derbyshire, England, where it occurs as hydrothermal vein fillings in limestone.
Is this blue john fluorite?
5-step field checkRun through these checks against the specimen in your hand. The more boxes tick, the more confident the ID.
- 1Test the hardnessTry to scratch blue john fluorite with a known reference. Blue John Fluorite sits at Mohs 4 — softer than the next harder reference, harder than the previous one.
- 2Check the streakDrag the specimen across an unglazed porcelain plate. Blue John Fluorite leaves a white streak.
- 3Read the lusterHold the specimen under a strong light. Blue John Fluorite typically shows a vitreous luster.
- 4Match the color rangeCompare against the expected color range: blue, violet, yellow, white.
- 5Look at form & habitCrystal system: cubic. Typical habit: massive.
Often confused with
Blue John Fluorite vs. its common look-alikes — and how to tell them apart in the field.
Often found alongside blue john fluorite
Minerals reported to co-occur with blue john fluorite. Spotting these in float or country rock is a strong cue you are in the right ground.
All properties
- Chemical formula
- CaF₂
- Mohs hardness
- 4
- Density
- 3.18 g/cm³
- Streak
- White
- Luster
- Vitreous
- Transparency
- Translucent
- Crystal system
- Cubic
- Crystal habit
- Massive
- Cleavage
- Perfect Octahedral
- Fluorescence
- Blue Under UV Light
- Rarity
- Rare
- Uses
- Lapidary, Ornamental, Collector
- Host rock
- Limestone Cavities
- Typical price
- $50-500 depending on specimen size and banding quality
Where rockhounds find blue john fluorite
Classic worldwide localities
- Castleton, Derbyshire, England
- Blue John Cavern, England
Field-hunting tip
Look in limestone cavities country — that is the host setting where blue john fluorite typically forms. If you start seeing calcite, galena, barite in float, you are in the right ground. Field specimens usually show a massive habit, so train your eye for that shape before scanning the outcrop.
Common questions
How do you identify blue john fluorite?+
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What rocks look like blue john fluorite?+
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Find blue john fluorite on the map
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