GAAJ Research Lab. Report - Natural Omphacite

Research Lab. Report

2009.09.17

Natural Omphacite

GAAJ-ZENHOKYO Laboratory
Makoto Okano (FGA, CGJ), Hiroshi Kitawaki (FGA, CGJ),
Dr. Ahmadjan Abduriyim (FGA), Dr. Jun Kawano

Deep green to black “jade” has been gaining popularity these past few years in the gem market in Hong Kong and China, and now it is appearing in the Japanese market as well. This material is often called “black jade” or “inky jadeite jade” but mineralogically this is not jadeite but omphacite.
The background and gemmological properties of omphacite are reported here.

♦ Introduction
Gem quality jadeite is not a single crystal but a rock consisting aggregated fibrous or granular crystals of jadeite. Jadeite forms solid solution, and therefore each jadeite grain has its individual colour, chemical composition or texture. Some of “jadeite” pieces for gem use also contain considerable amount of various minerals and some don’t qualify chemically as jadeite (Fukushima 2000). Recently deep green to black “jadeite” has become popular in Hong Kong or Chinese markets but this material is mineralogically omphacite (figure 1) (Ou Yang et al. 2003). This deep green to black omphacite has been imported in many cases to Japan, as “jadeite” which raised a concern, not only about confusion it may cause circulating the market, but also about correct understanding of this mineral even among the laboratory workers.

Fig. 1:

Omphacite

Fig. 2:

Jadeite

♦ Mineralogical classification of omphacite
  The name omphacite derives from the Greek omphax that means unripe grape. However, most of rock or gem material of omphacite displays deep green black colour and the pale green colour that is associated with the stone name is actually rare. Thus some may say the name originally belonged to prehnite or chrysoprase (Caley and Richards, 1956). Eclogite is a rock that is formed at high pressure deep under the ground and this mineral displays impressive colour combination of red of garnet and green of omphacite. Omphacite may also be contained in diamonds as an inclusion.
  Mineralogically omphacite, as with jadeite, belongs to the pyroxene group. Jadeite has ideal chemical formula NaAlSi2O6 while omphacite has chemical formula (Na, Ca) (Al, Mg, Fe) Si2O6, in which Ca replaced Na and Mg or Fe replaced Al. These two materials are separated according to the ratio between Na and Ca or Al and Fe. When Na/(Na+Ca) value falls between 0.2 and 0.8 and Al/(Al+Fe3+) value is over 0.5 the material is defined as omphacite (Clark and Papike, 1968), whereas a material with Na/(Na+Ca) value over 0.8 is defined as jadeite. Mineral species are still classified by such chemical composition today (Ou Yang, et al., 2003, and others). Pyroxene classification by chemical composition is shown in figure 3.
  Al component in jadeite may also be replaced by Cr, which produces green colour seen in stones from Myanmar, and when Cr replacement progresses and its content exceeds Al content the material is called kosmochlore.

Fig. 3:

Classification of pyroxene according to chemical composition (revised picture records from “special exhibition Jadeite” at the National Science Museum, Tokyo)

♦ Gemmological properties
Table 1 shows the gemmological differences between jadeite and omphacite.

	Jadeite	Omphacite
Colour	reen translucent	deep green translucent / opaque to black opaque
RI	1.66	1.66
agnification test	Fibrous texture is recognised	Features of natural origin are recognised (fibrous texture is not observed)
Spectrum test	“chrome line” is recognised	No absorption feature is observed (“chrome line” is not observed or indistinct)

Table. 1:

General identification features of jadeite and omphacite

1.	Colour - Jadeite (figure 2) commonly shows vivid green coloured by Cr, while omphacite generally shows deeper green to black due to Fe that is dominant over Cr (see again, figure 1). However, the black colour is actually observed as deep green against strong transmitted light such as a fibre optic light source (figure 4).
2.	R.I - Theoretical value of jadeite is 1.64-1.69 and that of omphacite is 1.66-1.72. Omphacite has higher value but practically these gemstones are typically cut in cabochon and their RI cannot be measured accurately down to the third decimal place. RI measurement by spot method often indicates the value for jadeite around 1.66 and for omphacite around 1.66 +.
3.	Magnification test - Jadeite shows aggregation of many crystals that is called “fibrous” texture (figure 5). Contrary to this, omphacite shows indistinct fibrous texture and under a gemmological microscope it shows very fine granular structure (figure 6).
4.	Spectrum test - Green jadeite is generally coloured by chromium and it shows an absorption line around 690 nm that is called “chrome line” on a handy-type spectroscope. Another absorption line originates in Fe is also seen around 437 nm that is called “jadeite line”. On the other hand, omphacite does not show the chrome line or the line may be indistinct if observed, and the jadeite line cannot be observed as it is cut off on a handy-type spectroscope.

As described above, jadeite and omphacite have differences in some points in standard identification tests, but there are some cases that the distinction becomes difficult and for definite identification more advanced analyses with sophisticated instruments are required.

Fig. 4:

A black omphacite revealed its true colour deep green under a strong transmitted light.

Fig. 5:

Jadeite shows “fibrous texture” due to aggregation of many crystals.

Fig. 6:

Omphacite shows very fine granular structure.