TBM71XXX: Group of Fluorite of the blue john variety, showing the characteristic concentric bands of yellowish and violet colours that have made this material famous throughout the world. The specimen displays a compact and well-defined structure, clearly showing the alternation of colour zones produced during the different stages of growth. A particularly interesting detail is the presence, on the upper part of the specimen, of small, well-individualized Fluorite crystals, a not very common feature that adds additional mineralogical interest to a material usually appreciated mainly for its ornamental value.

Geologically, Fluorite var. blue john formed through the crystallization of Fluorite in cavities and fractures within the Carboniferous limestones of the Castleton district. The characteristic colour bands reflect variations in the physicochemical conditions of the mineralizing fluids during the growth of the mineral.
Blue John Mine, Castleton, High Peak District, Derbyshire  England / United Kingdom

Specimen size: 8.7 × 5.8 × 4.4 cm = 3.43” × 2.28” × 1.73”