HomeTesting Jadeite Jade

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:

Visual examination of a jade-appearing material may yield significant identifying clues such as texture, surface luster, and fracture, as well as characteristic inclusions, evidence of dye, the presence of phenomena, and possibly other distinguishing characteristics. All these visual characteristics contribute to the typical appearance of a gemstone, thus allowing the gemologist with a well-trained eye to limit the range of possibilities quickly after an initial examination of the material. But even experts support the suppositions they make after a visual examination with standard gemological tests.

Among the characteristics of jadeite to note here concern its texture, surface luster, and fracture surface. In terms of its texture, it should be noted that jadeite is a very tough material. This is related to its internal structure. Differences in the structure of jadeite and nephrite, for example, can be seen under magnification: jadeite crystals appear as separate entities, while the crystals of nephrite appear to be woven together. This manifests itself visually, as noted by Hobbs (1982: 9), with jadeite looking granular and nephrite fibrous. Both jadeite and nephrite exhibit a slightly greasy luster, but nephrite tends to be greasier in its appearance than jadeite. Turning to the fracture surface of jadeite, Hobbs (1982: 10) describes jadeite and nephrite as exhibiting a "splintery fracture, which looks like the surface of a broken piece of wood." This characteristic, however, is more common with nephrite than jadeite. Unfortunately, several jadeite simulants also show similar characteristics. By and large, the value of visual examination is relatively limited in positively identifying jadeite. Hobbs (1982: 13) uses such terms as providing "valuable indications" and "suppositions" and concludes that these ned to be confirmed through gemological tests. We will look at the differences in appearance between jadeite and its simulants further in the section of simulants.

Jadeite's refractive index is about 1.66. Hall (1994: 124) gives it as 1.66-1.68, while Schumann (1997: 154) give it as 1.652-1.688. Field (2000: 3) reports that the mean refractive index of jadeite is 1.66 (alpha 1.654; gamma 1.667)" and notes that "this mean can be determined quite readily by the distant vision method." Read (1999: 281) states that "only [a] single vague shadow edge [is] visible on [the] refractometer at 1.66 due to [the] random orientation of crystal fibres." Hobbs (1982: 13) states that "the refractometer is one of the most helpful instruments in separating jadeite from its simulants." This is because almost all of these simulants have refractive indices that are significantly different than jadeite's. The problem is that jadeite and most of its simulants are usually cut with a round surface in such a way that their shapes make it difficult to obtain readings with a refractometer. This necessitates using the "spot technique" or "distant vision method." Hobbs (1982: 13) describes the spot technique as follows: "The spot technique requires that a portion of the curved surface be placed or held on the refractometer with a small drop of liquid, the size of which is reduced until the image that is seen without the eyepiece magnifier is only two or three scale increments."

Jadeite is doubly refractive. According to Field, the birefringence is 0.013. Other sources give somewhat different numbers: Hall (1994: 124) gives 0.012 and Schumann (1997: 154) gives 0.020. However, Hobbs (1982: 13) warns that "it is rare to see the full spread of refractive indices listed on the property chart because" jadeite is a crystalline aggregate and "only one refractive index is easily resolved with the spot technique." To obtain a birefringence reading, Hobbs (1982: 14) recommends using the birefrengence blink technique that involves rotating a polaroid plate in front of the refractometer. This technique is illustrated and described by Hobbs (1982: 13, fig. 13).

Jadeite has a specific gravity of 3.33-3.35. Field (2000: 3) reports that "most jadeite...will remain suspended or very slowly sink in methylene iodide (di-iodomethane) that has a density of about 3.32-3.33 at normal room temperature." Hobbs (1982: 15) also recommends using methylene iodide when testing for jadeite and warns that "jadeite, and many jade-like materials, may contain impurities that will cause the specific gravity to vary. Hobbs (1982: 15) also notes that while three common jadeite simulants (grossularite, zoisite, and idocrase) have specific gravity values that can be confused with jadeite's all of them have refractive indices that are a good deal lower than jadeite's.

Spectroscopic analysis is a useful means of identifying jadeite. Moreover, as noted by Hobbs (1982: 15), "the spectroscope is helpful in that both cut and rough, as well as mounted or loose, materials can be tested." Read (1999: 281) discusses the appearance of jadeite when examined with a spectroscope (also see Hobbs 1982: 15-17; Webster 1975: 228; Walker 1991: 39-40). He states that there is a "diagnostic line in the blue; 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)." Field (2000: 3) adds additional detail:

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 colour to transmit well. 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.

Huang (1999) provides data on the characteristics exhibited by jadeite when examined with a Raman spectroscope:

The Raman 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 is studied along with fourteen other gem minerals and Huang provides a flow chart (page 311) showing identification procedures to separate one mineral from another. We shall return to the question of identifying dyed jadeite below in the section of treatment of jadeite.

Both Hobbs (1982: 18) and Walker (1991: 41) note that the most precise test in jadeite identification involves X-ray diffraction by the powder method. However, as both authors point out, unfortunately this method is feasible only for sophisticated laboratories.

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