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Chemical composition Beryllium sillicate


Crystal system Trigonal
Habit Tabular to prismatic
Fracture Conchoidal (brittle)
Cleavage Indistinct (one direction)
Hardness 7.5 - 8
Optic nature uniaxial +
Refractive index 1.651 - 1.670
Birefringence 0.016
Dispersion Low, 0.015
Luster Vitreous
Specific gravity 2.92 - 2.97
Pleochroism Seen in strongly coloured stones
Fluorescence Sometimes pale greenish / blue or pale rose
A facetted colorless gem quality phenakite.
Photo courtesy of Chaman Golecha, Gem Testing Laboratory, Jaipur
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A colorless gem quality phenakite crystal.
Photo courtesy of Jeffrey Hunt


Trigonal system.
Habit: Tabular, prismatic, rhombohedral, acicular.





Colorless; also yellow, pink, brown, pinkish red due to surface stains.


Phenakite is uniaxial with a positive optic sign.
Refractive index range: nω = 1.651-1.654 and nε = 1.663-1.670.
The full birefringence is 0.016.


Not many gemstones are uniaxial + (zircon, quartz, phenakite and some idocrase). Finding the optic sign with the use of a retardation filter should narrow down the possibilities.
Quartz will almost always show a typical bull's-eye pattern (or Airy spirals).


Nothing diagnostic.


Phenakite sometimes shows pale greenish / blue or pale rose fluorescence in UV light.
Under X-ray it fluoresces blue.


Dichroism in observed in strongly colored stones.


May have many needle inclusions; 2-phase; fingerprints with phase inclusions.


Name Key seperation
Spodumene Magnification features, UV, biaxial figure, cleavage and S.G
Sillimanite S.G., R.I., birefringence, biaxial Figure
Rock Crystal Lower S.G. (floats 2.88), bulls eye optic figure, R.I.
Topaz More heft (sinks 3.33), biaxial figure, R.I.
Zircon S.G., R.I., birefringence


Geological occurrence: Commonly in pegmatites, mica schists, in granite drusses.

Geographical locations: Tokajawa river, Ural Mountains with emerald and chrysoberyl; Miask district, Rissia; large colourless crystals at San Miguel di Praciavaba, Minas Gerais; Tanzania; Maine, New Hampshire & Colarado, USA; Switzerland; Austria; Alsace, France; Namibia; Norway and Sri Lanka (large stone).