Blue John | Gemopedia

Blue John is the banded purple and white or yellow variety of fluorite. The major source of this color-zoned fluorite is Castleton in Derbyshire, England, where it is found in at least 14 differently patterned veins.
Colors
Bluish Violet To Purple Bands In A Reddish To Colorless Background

Blue John Classification

Common Name

Blue John

Species

Fluorite

Variety

Blue John

Colors

Bluish Violet To Purple Bands In A Reddish To Colorless Background

Alternate Names

Gemstone Groups

Key Separations

Appearance, refractive index, specific gravity and possibly cleavage

Comments

Blue John was first found in the Blue John Cavern in Castleton, Derbyshire, England.

Blue John Optical Properties

Transparency

Transparent To Translucent

Refractive Index

1.434-1.434
Tolerance: (+0.001/-0.001)

Birefringence

Optic Character

NA

Optic Sign

NA

Polariscope Reaction

Singly Refractive (SR)

Fluorescence

SWUV: Variable
LWUV: Variable

CCF Reaction

Pleochroism

None

Dispersion

Strength: weak fire Value: 0.007

Comments

Blue John Chemistry & Crystallography

Chemical Name

calcium fluoride

Chemical Formula

CaF2

Synthesis

Crystal System

Cubic

Classification

Halide

Nature

Natural

Crystallinity

Crystalline

Comments

Blue John Characteristic Physical Properties

Hardness

4

Streak

White

Specific Gravity

3-3.25 Typical: 3.18

Toughness

Poor

Inclusions

Blue John will sometimes have triangular negative crystals, mineral inclusions, color zoning, healing cracks 2-phase and 3-phase inclusions and liquid inclusions. Bluish violet to purple bands in a reddish to colorless background. Might have pits or scratches. Look to see if the stone has been impregnated with resin.

Luster

Vitreous

Stability

Fair

Fracture

Conchoidal, Step-Like

Cleavage

Perfect, in four directions

Comments

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