GAAJ Research Lab. Report - Natural Omphacite

♦ EDXRF analysis (JEOL, JSX-3600)
EDXRF analysis was done on jadeite and omphacite. Genuine jadeite consists of the major elements Na, Al and Si, but jadeite stone for gem use is an aggregate of fine crystals and contains a trace of various minerals. We have once confirmed in our analysis with EPMA that even a high quality lustrous emerald-green jadeite contained a small amount of omphacite between the crystal grains. Thus the elements such as Mg, Ca or Fe in several hundreds ppm to several percents may often be contained as trace components.
Omphacite for gem use has higher content of MgO and CaO than jadeite and their amounts of mol % are as much as that of Na2O. The amount of mol % of Na/(Na +Ca) in the jadeite analysed in this study was around 0.9 and in the omphacite it was between 0.6 and 0.7.

Fig. 7:

Result of EDXRF analysis / jadeite (left) and omphacite (right)

♦ Infrared spectral analysis (SHIMADZU, IR Prestige-21)
  The jadeite and omphacite were analysed by FTIR. No difference between jadeite and omphacite was recognised in the data of middle to long wavelength region with transmitted beam. Then, data was obtained in shorter wavelength region under 1500 cm-1 with reflected beam on the surface for comparison (figure 8).
  Jadeite has an absorption end at 1250 cm-1 on the longer wavelength side and shows acute absorption peaks at 990, 860, 680, 650, 550, 490, 440 and 410 cm-1.
  On the other hand, omphacite has an absorption end at around 1220 cm-1 on the longer wavelength side and shows sharp absorption peaks at 1020, 905, 670, 630, 540, 470 and 420 cm-1, all of which are located differently from those of jadeite.

Fig. 8:

Result of infrared spectral analysis / jadeite (green) and omphacite (red)

♦ Micro Raman spectral analysis (Ranishaw RAMASCOPE SYSTEM1000)
Jadeite and omphacite were analysed by micro Raman spectroscope (figure 9). Jadeite shows the main peak at 700 cm-1 with rather weaker peaks at 1035, 985 and 370 cm-1.
Omphacite shows the main peak at 680 cm-1 with other weaker peaks at 1020 and 560 cm-1, which are all different from the peaks of jadeite. The features of broad peaks centred at 400, 375 and 345 cm-1 are seen only in omphacite.

Fig. 9:

Result of micro Raman spectral analysis / jadeite (green) and omphacite (red)

♦ Nomenclature of omphacite on a gem identification report
  Gemmological laboratories in the major “jadeite” consuming countries such as China or Hong Kong had no unified identification standard for jadeite or diamond and each laboratory used its own identification methods and nomenclature.
  As a result, quite a bit of confusion was caused in the jewellery industry. In August 2006 the Gemmological Association of Hong Kong issued “Standard Methods for Testing Jadeite Jade for Hong Kong” through discussion and collaboration among six major laboratories in Hong Kong and it was registered as ISO/IEC 17025:1999 on a reference manual of Hong Kong Laboratory Accreditation Scheme (HOKLAS).
  Jadeite forms solid solution with omphacite or kosmochlore and each of them coexists or aggregates to make up a rock. Such rocks are difficult to identify only by colour and appearance and the Gemmological Association of Hong Kong proposed the Chinese name for jadeite “FeiCui” as family name for jadeite, omphacite and kosmochlore. Thus a stone of these materials is described as “FeiCui” as variety name on the result column of gem identification reports issued by laboratories in Hong Kong, and it is required to describe a sub type among jadeite, omphacite or kosmochlore on the remarks column.
  International gemmological laboratories in the U.S.A., Switzerland, Italy or Thailand, affiliated with LMHC, clearly distinguish between jadeite and omphacite as variety names. When a stone is omphacite the name is definitely described on a result column of a gem identification report issued by these laboratories. Similar to these international laboratories, also in Japan, omphacite is described as follows:

Species:	Natural omphacite
Variety:	Omphacite

♦ Summary
Deep green to black “jade” has been popular these past few years in the gem markets in Hong Kong and China and this material has also been appearing in the Japanese market. This material is often called “jadeite” but mineralogically this is not jadeite but omphacite, which is a mineral that belongs to the pyroxene group and forms a solid solution with jadeite and kosmochlore. The distinction between them requires a compositional analysis by EDXRF analysis, but a rough separation can be made by infrared spectral analysis or Raman spectral analysis. International gemmological laboratories make clear distinction between omphacite and jadeite when describing its name on a gem identification report.

References
1) Hideaki Fukushima (2000) Various issues on jadeite. GEMMOLOGY Vol.XXXI, No.375. pp. 15-17.
2) Ou Yang Chiu Mei, Li Jian Qi, Li Hansheng and Bonita Kwok (2003) Recent studies on inky black omphacite jade, a new variety of pyroxene jade. The journal of Gemmology, Vol.28. No.6. pp. 337-344.
3) Caley and Richards (1956) Theophrastus On Stones.p. 51, 120-121.
4) Clark and Papike (1968) Crystal chemistry of Omphacites, American Mineralogist, 53, pp. 840-846

For further information, please contact:
GAAJ-ZENHOKYO Laboratory
E-mail: laboratory@gaaj-zenhokyo.co.jp

PreviewPage