GEMLAB REPORT:
January 1998 - Issue #7
By: Ted Themelis
In This Edition:
- Emerald or Green Beryl?
It is well-known fact that many gem dealers, jewelers, and
consumers sometimes confuse light green emeralds with green
beryls. The issue of determining their true identity is still as
controversial and much debated, as deciding what is ruby and
what is pink sapphire (see Gemlab Report December 1997) or what
is pink tourmaline and what is rubellite (forthcoming Gemlab
Report - February 1998 issue).
In the case of light green emerald versus green beryl, the
"transition point" needed to distinguish clearly one material
from another is yet to be defined precisely and accepted
universally. Chromium is the major transition element (in the
form of color-bearing impurities) causing the green coloration in
beryl. When chromium oxide replaces a few alumina atoms in the
basic beryl structure Be3Al2 (SiO3)6, under suitable conditions,
the result is green coloration. Beryls with that coloration show
distinct chromium absorption lines and/or bands in the visible
portion of the electromagnetic spectrum, as follows: The ordinary
ray (o-ray) shows a strong “doublet”, at 683nm/680nm; an
absorption line at 637nm; an absorption band from 625nm to
580nm; and, seen only in chrome-rich emeralds, absorption lines
at 478nm and 472nm. The extraordinary ray (e-ray) shows a very
strong “doublet” at 683nm/680nm and two weak absorption lines at
662nm and 646nm.
Many gemologists and other members of the gem and jewelry
industry consider the presence of spectral chromium absorption as
the primary deciding factor of justifying the use of the term
"emerald". The precise determinations of these spectral
absorption lines and /or bands require the availability of a gem
spectroscope with high resolution and an experienced
spectroscopes who will be able to detect these lines/bands with
some accuracy.
Typical localities of light green emeralds and green beryls are
Colombia, former USSR, Brazil, India, Pakistan, Zambia,
Madagascar, Tanzania, and elsewhere. The cause of color in these
beryls may be due to chromium or to vanadium, or to
chromium/vanadium. Some beryls from Brazil, (Pilao Arcado and
Bahia) are colored mainly by traces of vanadium. According to R.
Webster, author of “Gems”, these beryls should be called “green
beryls” (Gems, 4th ed. page 107), despite their pale emerald
coloration.
Wood and Nassau, on their article “The Characterization of Beryl
and Emerald by Visible and Infrared Spectroscopy”, published in
The American Mineralogist (1968), vol.53, pp. 777-800,
demonstrated that certain vanadium-colored Brazilian beryls and
chromium-colored emeralds, appear comparable in final color,
since they have similar absorption spectra. Wood and Nassau also
noted that many beryls from various localities contain both
vanadium and chromium in varying minute amounts. They suggest
that, if chromium is present to about 0.1 percent or more, the
beryl shall be called “emerald” and another designation shall be
given vanadium-colored beryls. This, of course, would require
non-destructive geochemical analysis on the beryls in question.
The majority of gem dealers are judging problematic beryls
primarily by their green hue -and those with the characteristic
"emerald-green" appearance are called emeralds. Some gem dealers
contend that, if a Colombian green beryl has typical three-phase
inclusions (gas, liquid, solid) regardless its light green
appearance, it should be called emerald, because it comes from a
confirmed emerald deposit and it has the typical characteristic
"emerald-type" inclusions. Other gem dealers and gemologists
distinguish green beryl from light green emerald not only by its
hue and saturation, but primarily by its tone; this method
considers that if the tone is belo 30% in the gray scale
(100%=black), then the stone should be called green beryl;
otherwise it should be called emerald. Other gem dealers consider
the tone and saturation, both as the deciding factors in
distinguishing green beryl from light green emerald; however,
they do not justify the emerald designatio on beryls which have
characteristic three-phase inclusions.
The issue is more complicated when fractured light green beryls
are routinely “oiled” with oils, resins, and oleoresins mixed
with green organic dyes (see Gemlab Report Oct.1997 issue).
Eventually, when the dye dries out and the true color of the
beryl is revealed, its value dramatically drops, causing many
problems to sellers and to the buyers alike. As anyone has
guessed by now, there is no universal agreement on this
controversial issue. As a result, in most instances green beryls
are traded as light green emeralds, and, rarely, vise- versa.
Since the price of emerald is steadily increases, the
emerald/green beryl problem grows accordingly and proportionally.
Universally accepted terminology is urgently needed.