Lets Talk Gemstones
By Edna B. Anthony, Gemologist
(Contact the author for permission to reproduce this
article in any form.)
P.O.# 62653; COLORADO SPRINGS, CO. 80962
Until the early nineteenth century, certain crystals found with emerald in
the mines on the Tokovaya River (east of Sverdlovsk in the Ural Mountains)
were presumed to be quartz. In The Illustrated Encyclopedia of Minerals and
Rocks, Dr. J. Kourimsky states that, during an inspection of the mines, the
Russian inspector and mineral collector, Count Petrovsky, collected several
specimens. A closer examination of the material revealed properties different
from those of quartz, and further analysis identified beryllium silicate.
An unknown mineral was discovered! Phenakite’s resemblance to quartz caused
the use of the Greek word meaning “deceiver” to be adopted as the name.
Phenakite is a member of the phenakite group of the nesosilicates. Phenakite
develops primarily in pneumatolytic or hydrothermal environments, but the
decomposition of beryl can produce it as a secondary mineral. Its short prismatic,
acicular or frequently twinned, typically flat rhombohedral crystals develop
in the hexagonal crystal system. Fibrous spherulites and granular forms also
occur. A single direction of cleavage is indistinct. Although faceted gems
seldom exceed the five-carat range, its usual pegmatitic origin does allow
the development of rare, large, usually heavily included, transparent to
translucent crystals. [The largest known was a 1470-carat pebble found in
Sri Lanka. An eye-clean 569-carat faceted oval gem, inhabited by numerous
tiny acicular inclusions, plus several smaller gems were cut from this find.]
Less common environments for phenakite are mica schists and granites. The
best known deposits are located in the Ural mountains, Kragero in Norway,
San Miguel de Paracicaba in Brazil, Pala County in California, Lord’s Hill
in Maine, Colorado’s Mount Antero and the Pike’s Peak area, Virginia and
New Hampshire in the USA, and Italy, Sri Lanka, Tanzania, Namibia, and Zimbabwe.
Associated minerals include albite, amazonite, apatite, beryl, chrysoberyl,
mica, quartz, fluorite, and topaz. Care must be taken not to mistake it for
rock crystal, beryl, topaz, sapphire, or diamond.
Brazil is noted for its large, colorless, eye-clean, cuttable phenakite material.
Extremely rare red crystals from the Urals yield unique gems rarely seen
and seldom available. Though surface stains are responsible for the pink,
pinkish red, brown, and yellow hues, as well as the impurities that produce
colors in some crystals, phenakites are remarkably chemically pure. Inclusions
of other minerals seldom occur. Numerous needle-like inclusions can produce
a translucent crystal that, when properly cut, displays an attractive chatoyancy.
Jaroslav Bauer and Vladimir Bouska caution in A Guide in Color to Precious
and Semi Precious Stones that the typically delicate colors of phenakite
are often light-sensitive and may fade in as little as a few months if frequently
exposed to sunlight. Deep-toned phenakite gems usually exhibit strong pleochroism.
The brilliant cut displays phenakite’s attractive gem attributes well. Phenakite
is a bright gem, but its weak dispersion limits the fire and scintillation.
However, a hardness of 7.5 - 8 and its toughness makes phenakite a durable
choice for any type of jewelry for a collector of unusual gems.
Composition Be2[SiO4] beryllium silicate
Variety colorless and by color
Crystal System hexagonal (per Arem and Schumann; trigonal
(per Bauer and Bouska)
Habit crystal-rhombohedral, prismatic, acicular, granular,
Cleavage imperfect; in one direction
Fracture conchoidal; brittle
Fracture Lustre vitreous
Diaphaneity transparent, translucent
Colors colorless, brown, pink, red, yellow
Specific Gravity 2.93 t0 3.0
Hardness 7.5 to 8.0
Refractive Indices o= 1.654; e= 1.67
Optic Character uniaxial positive
Pleochroism distinct; colorless: orange-yellow; blue: red-strong
Luminescence pale green or blue, possible light rose in
Spectrum not diagnostic
Chelsea Filter not applicable
Aqua Filter not applicable
Solubility no information
Thermal Traits avoid thermal shock; infusible; [Manuel
of Mineralogy: produces white enamel when fused with sodium carbonate
Treatments none known
Inclusions aikinite crystals, mica in material from Brazil