The Science of Jade
The pyroxenes are perhaps one of the most complicated groups of minerals. Here is a summary of the multi-faceted properties of jadeite, its identification, classification, and qualifications according to science.
Composition and Properties
Jadeite is a silicate of the subdivision inosilicate. Among this subdivision, inosilicate, is a group of 24 minerals named pyroxines. Jadeite is one of these pyroxines. The chemical formula for pure jadeite, (Jd100), is: NaAl(Si2O3)2. Pure jadeite is colorless and transparent in its purest form. Although because of light refraction from its texture and minute aqueous inclusions, it appears “white”.
Jadeite can be nearly any color. Though the color green is the most commonly associated color of jadeite, blues, reds, yellows, oranges, greys, black, rainbow, browns, and violets can be found. This color variation is due to the introduction of other pyroxenes. These are usually: diopside, kosmochlor, hedenbergite, and aegirine. Jadeitic pyroxenes usually constitute at least 90-95% of the rock in which it is found. Other minerals that can be found in jadeite include sodic amphibole, albite, analcime, tremolite, clinochlore, banalsite, and chromite. In some jadeite formation, fibrous tremolite and actinolite may be infused into the rock. This causes polymineralic jadeite which can have colors of white with gray-green to blackish specks or streaks. It is rare for this type of jadeite to be green and exhibit even color distribution.
Only a small amount of minor elements are required to produce vivid colors in jadeite.“Imperial jade” color, for instance, is due to the presence of chromium (Cr), which absorbs blue and green light. Blue-green, bluish black, and blue-black jadeite are due to the presence of iron (Fe). The darkest colors contain a high percentage of iron oxide (FeO) and resemble a pyroxene called omphacite. The term “leek green” is commonly applied to aggregates of jadeite and sodic amphiboles. The violet color is attributed to a Fe2+—O—Fe3+ intervalence charge transfer in nearly pure jadeite. The mauve color in jadeite is due to the presence of manganese.
Jadeite is commonly found in mines and river beds, alongside serpentine, nepheline, calcite, quartz, aragonite, glaucaphane, and vesuvianite. It is important to know the differences between these other specimens for proper identification.
Jadeite crystals are in a monoclinic formation, which is composed of fine-grained, fibrous, highly inter-grown, interlocking crystals. These inter-grown crystals give this gemstone its hardness. When jadeite is fractured, it is splintery and brittle. Its transparency can range from
translucent to opaque. It has a Mohs hardness measurement of 6.5- 7.0. In a microscopic comparison, nephrite has very fibrous looking crystals, while jadeite crystals appear independent. Though nephrite is softer than jadeite, nephrite is actually a bit more resilient due to the fibrous flexibility of its crystals.
Jadeite’s appearance can vary greatly according to where it was formed. Mesoamerican jadeite and Burmese jadeite, for instance, have different compositions. This makes Burmese jadeite appear different at times. American jadeite tends to be less translucent than Burmese jadeite, though tends to be much more dynamic in its color variety and color pattern.
Categories of Jadeite
There are several ways that scientists and gemologists have to group or categorize the jadeite found in Central America. It can become difficult to categorize the many mixtures of jade. This is one way to do it.
The composition of Mesoamerican jade can be divided into seven groups: 1) Motagua Light: light green color; 2) Motagua Dark: green-black color (basically omphacite); 3) Maya Green: emerald green; 4) Costa Rican Light; 5) Costa Rican Dark; 6) Chichén Green; and 7) Miscellaneous.
The first two groups are easily differentiated chemically from the group of samples named “Maya Green”, which have significantly higher chromium values. There are also significant differences in the cobalt content in the three categories of Guatemalan jade stone. Overall the stones are characterized in these following categories:
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.
The Maya Green samples possess abundant jadeite, trace omphacite, and relatively low abundances of albite, muscovite, and analcite.
The methods of identifying jadeite are listed as follows: visual examination, refractive index readings, specific gravity determination, spectroscopic analysis, hardness tests, Chelsea filter observation, and X-ray diffraction.
A visual examination of jadeite includes: texture, surface luster, fractures, as well as characteristic inclusions, evidence of dye, the presence of phenomena, and possibly other distinguishing characteristics that a well-trained eye can notice. This visual examination will determine which tests will need to be performed for identification.
The refractive index of jadeite is, depending on your source, between 1.652- 1.688. The refractometer is one of the most helpful tools to separate jadeite from its simulants. Because of the way jadeite is usually cut, such as in cabochons and other round shapes, the “spot technique” is needed to read the refractometer correctly. A flat surface such as a roughly cut jadeite face can suffice to avoid this technique.
The specific gravity of jadeite is 3.33- 3.35. Three common jadeite simulants have similar specific gravity; grossularite, zoisite, and idocrase. To differentiate between these, test for their refractive indices.
Spectroscopic analysis is very helpful for the identification of jadeite as it can be used on rough, cut, loose or mounted specimens. When examined with a light spectroscope, jadeite has a diagnostic line in the blue; while chrome-rich jadeite has a doublet in the red, and two bands in the red-yellow. Stained jadeite has a band in the orange and one in the yellow-green, plus the diagnostic line at 437 nm. Green jadeite shows several bands in the violet, the strongest being at 437 nm. It is intense enough to be discerned by reflected light and by transmitted light if the material is not too opaque or too dark in color. Naturally green jadeite also shows three chromium lines somewhat resembling steps or louvres in the red, at about 630, 660 and 690 nm; but above this is a light zone from about 670 to the end of the visible spectrum. In the “natural green” spectrum just described, there is nothing but darkness above the 690 nm band. Note however, that the band at 437 nm is present in both the natural and dyed examples. The Raman Spectroscope modes of jadeite are 292 and 328 cm-1 (Na-O stretching mode); 374, 416, 434 and 576 cm-1 (Al-O vibrational modes); 524, 700, 779 cm-1 (Si-O bending modes) and 887, 986, 992 and 1040 cm-1 (Si-O stretching modes). There is little variation in the wave number of Raman modes with substitution of iron and chromium in jadeite. Slope of the variation is negative with increasing substitution of iron and chromium.
Jadeite has a Mohs hardness of 6.5 to 7.0, as previously mentioned. Jade such as nephrite is softer at 6.0 to 6.5. A common field test for jade is attempting to scratch the surface with a pocket knife. Standard stainless steel is softer than jade, hence, jade will not scratch. Also, a standard hardness testing kit will suffice. This kit may use stones such as feldspar (6 hardness), quartz (7), and topaz (8) to deduce hardness grade.
When looking through the loupe-like Chelsea filter, jade will show a pinkish red reaction. Although be aware: some enhanced jade will display similar colors. As other tests, this test should be used in conjunction with a complementary test to confirm the identity of jade.
The most precise test in jadeite identification involves X-ray diffraction by the powder method. However, this method is only feasible with sophisticated laboratory equipment.
In summation of the identification of jadeite, “an understanding of jadeite is not limited to the technical or exacting, but it also requires a feeling for the cultural, textural, and ephemeral qualities that make the study of jade unlike any other in the world of gemstones.” Hughes, Galibert, et al (2000: 2)
What causes the colors of jadeite?
Ice jadeite is jade without chromophores, or impurities.
Dull greens, blue-greens, bluish black and blue-black jadeites are related to the presence of iron. The darker of these colors contain a relatively high percentage of iron oxide. These closely resemble omphacites. Though these colors are mostly of Burmese origin, these principles still apply to Guatemalan jade.
Dark green streaks are sometimes caused by staining from iron compounds.
Green-black jadeite is rich in iron oxide with a mixture of diopsides, aegirine, and jadeite and contains a high proportion of iron oxide.
Black jade can be opaque, fine grained, to translucent, coarser grained. This color jade ca n be the hardest jade at 7 on the Moh’s harness scale. It is 95% jadeite whose color is derived from the 5% presence of black or dark opaque dust-like materials that are associated with minute inclusions of metallic oxides and sulfides, amorphous carbon, organic salts, water, CO2, and various hydrocarbons distributed throughout the stone. Black jadeite is an aegirine-augite rich, metamorphic rock. It can also contain tarramite, titanite, albite, analcite, grossular, and white mica. This rock is very durable, takes a very good polish, and shows little grain definition. Guatemalan black jadeite represents the creamiest, richest, and best black jadeite in the world.
Grey or black is caused by graphite staining. This staining is usually only in Burmese jadeite
This jade and jadeite summary was modeled after the extensive and superbly written article by: Kim Be Howard, A.G. (C.I.G.), Surrey, B.C.http://www.cigem.ca/431.html