JADEITE - Canadian Institute of Gemmology

in the cobalt content in the three categories of stone. Overall they characterize the stones in these
categories as follows (page 43):
The Motagua Light samples can be characterized as consisting of major abundances of
jadeite and albite, with occurrences of prargonite and analsite... The Motagua Dark
specimens contain less abundant jadeite, major abundances of omphacite, and variable
amounts of analcite. In contrast, the analyzed Maya Green samples possess abundant
jadeite, trace omphacite, and relatively low abundances of albite, muscovite, and analcite.
Data are provided about the chemical composition of a number of the stones belonging to these
three categories (page 45). A sample of eight Motagua Light stones have the following
characteristics: 58.91% Si0 2, 24.6% Al 2O 3, 12% Na 2O, 1.97% CaO, 1.29% MgO, 1.01% FeO, and
traces of Cr 2O 3, K 2O, and MnO. A sample of seven Maya Green stones have the following
characteristics: 57.5% Si0 2, 20.0% Al 2O 3, 10.7% Na 2O, 4.78% CaO, 3.84% MgO, 1.20% FeO,
0.32% Cr 2O 3, and traces of K 2O and MnO. A sample of five Motagua Dark Omphacite stones have
the following characteristics: 53.8% Si0 2, 14.4% Al 2O 3, 6.2% Na 2O, 10.36% CaO, 7.46% MgO,
2.32% FeO, 0.10% Cr 2O 3, and traces of K 2O and MnO.
On the basis of electron beam microprobe analysis the authors plot the relative jadeite
composition in the various samples (see fig. 2.3, page 49). Among the findings is that: "The Costa
Rican Light specimens all lie near the pure jadeite corner, and the Maya Green are close to the
jadeite-omphacite boundary." Chichén Green falls in between. Later in their chapter (page 58), the
authors discuss the distinctiveness of the Costa Rican samples, which have a tendency towards a
bluish-green color (like the so-called Olmec pieces), from those found elsewhere. Their
distinctiveness mineralogically is related to "the virtual absence of mica and the low albite content in
the Costa Rican specimens."
Tests. Let us now turn to some of the tests that can be used to identify jadeite. Hobbs (1982)
lists several tests: visual examination, refractive index readings, specific gravity determination,
spectroscopic analysis, hardness tests, and X-ray diffraction. To this could be added the use of a
Chelsea filter. In regard to the latter, it is interesting to note that while green jadeite's color is derived
from the presence of chromium, it "does not show red under the Chelsea filter, nor does it do so
under either LW or SW UV light" (Field 2000: 3). Under long-wave ultra-violet light "the paler
coloured green and the yellow, mauve and white jadeite shows a whitish glow of low intensity, the
darker coloured jadeite being unresponsive." Among the tests discussed by Hobbs, we will review
all except for hardness tests since such tests are rarely used for jadeite. Hobbs (1982: 18) indicates
that such tests are not very useful for jadeite and "would only help separate materials that have a
hardness value that is significantly lower than jade, such as serpentine, calcite, and talc."
Turning first to visual examination, Hobbs (1982: 6-7) remarks:
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