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by gagan choudhary

A New Type of Synthetic Fire Opal – “Mexifire”


Authors: Gagan Choudhary and Rajneesh Bhandari


(This article was first appeared in Gems & Gemology, Vol. 44, No.3, pp 228 - 233)

Background

Natural fire opal is known mainly from Mexico (see e.g.Spencer et al., 1992). Other locations include Kazakhstan, Turkey (O’Donoghue, 2006), Ethiopia (Johnson et al., 1996), Oregon (Laurs and Quinn, 2003), and Java (Sujatmika et al., 2005). The high value and commercial interest in fire opal from various locations has stimulated the production of synthetic counterparts.


Natural opal is hydrated silica, amorphous to microcrystalline, with the chemical formula SiO2·nH2O Webster 2002). Soon after the structure of opal was determined, in 1964, the first attempt at manufacturing synthetic opal was reported (Smallwood, 2003). It was first introduced commercially in 1975 (Smallwood, 2003).


Synthetic opal is produced by a number of sources, including Gilson, Kyocera/Inamori, and some Russian manufacturers (e.g., Quinn, 2003; Smallwood, 2003). Several varieties are available, with or without play-of-colour, ranging from white and black to pink, orange, or brown. This article describes the properties of a new type of synthetic fire opal (figure 1) developed by one of the authors (RB) and marketed as “Mexifire.”


Most of the synthetic opal produced in the past showed distinct play-of-colour, which was caused by a three-dimensional array of uniformly sized particles (Nassau, 1980; Schmetzer, 1984; Smallwood, 2003). These synthetic play-of-colour opals can be achieved by: (1) producing suitably uniform sized silica spheres; (2) settling these spheres into close-packed structure; and then (3) solidifying, aggregating, dehydrating, and compacting the array into a stable product. The Mexifire synthetic opals do not exhibit play-of-colour and are made using a different process (modified sol gel). Under specific conditions, silica precursors (tetra ethyl ortho silicate [TEOS] in this case) are used to produce a matrix of silica, which is similar to the structure of natural fire opal. As in natural fire opal (Fritsch et al., 1999), the orange colour is caused by traces of iron. Unlike natural opal, these synthetic opals do not craze.





















Figure 1: These samples (0.23–3.50 ct) of synthetic fire opal, marketed as “Mexifire Opal,” were studied for this report

Materials and Methods

We examined 38 faceted fire opals: 26 synthetic (0.23–3.50 ct; again, see figure 1) and 12 natural that were said to be from Mexico (0.60–4.00 ct; e.g., figure 2). Only a limited range of sizes for synthetic samples is reported here, but larger pieces can also become available. Standard gemmological tests were performed on all samples. Refractive index was measured using a GemLED refractometer. Hydrostatic specific gravity was measured using a Mettler Toledo CB 1503 electronic balance. A polariscope was used to check for strain patterns. Fluorescence was observed to long-wave (366 nm) and short-wave (254 nm) UV radiation. Absorption spectra were observed with a desk-model GIA Prism 1000 spectroscope. We examined the internal features of the samples using both a binocular gemmological microscope (with fiber-optic and other forms of lighting, including darkfield and brightfield) and a horizontal microscope with the samples immersed in water.

Figure 2: Among the natural fire opals studied for a comparison to the Mexifire synthetics are these brownish orange (left, 0.30 – 0.86ct and orangy yellow samples (right; 0.67 – 0.81ct, all of which are reportedly from Mexico

Results and Discussion

Visual Characteristics. The Mexifire synthetic opals ranged from medium-to-dark brownish orange to orangy yellow (again, see figure 1). Of the two sets of natural fire opal that were tested for comparison, one set had a similar brownish orange colour (again, see figure 2, left) while the other was a brighter orangy yellow (figure 2, right). All the synthetic samples were evenly coloured when viewed from the table. When viewed from the side, one of them was darker in the girdle area than in the pavilion (figure 3). This colour variation also has been seen in natural fire opal (see Gübelin and Koivula, 2005, p. 498). All synthetic and natural fire opals appeared transparent under normal viewing conditions however displayed slight haziness when observed using fiber optic light. All synthetic samples took a good-quality polish while few natural samples exhibited a dull subject to the usage since these comprise the master set and are being used regularly (which caused some abrasions and scratches).










Figure 3: One of the synthetic fire opals displayed variations in bodycolour when viewed from the side. The girdle area appears darker than the pavilion. Brightfield illumination; magnified 15x

Gemmological Properties. The gemmological properties of the synthetic and natural fire opal samples are described below and summarized in table 1.


Table 1: 
Click to edit table header
 
 
Property
               Mexifire synthetic fire opal
  Natural fire opal
Colour
Brownish orange to orangy yellow
Brownish orange to orangy yellow
Colour distribution
Typically even
Often colour zoned; flow-like or wavy pattern
Diaphaneity
Transparent under normal viewing conditions, while        translucent/ turbid with fibre-optic light
Transparent to translucent
Quality of polish
Good
Good to dull
Polariscope reaction
Strong strain pattern with snake-like bands
Weak strain pattern; no snake-like bands seen
Refractive index
1.380–1.405
1.400–1.435 (this study)

1.420–1.430 (O’Donoghue, 1988)

1.440- 1.460 (O’Donoghue, 2006)
Specific gravity
1.63–1.77
1.92–2.06 (this study)
2.00 (Webster and Read, 2002)
UV fluorescence
Inert
Inert
Desk-model spectroscope
No features
No features
Internal features
1. Turbidity following zones
2. Scattered pinpoints
1. Turbidity following zones
2. Scattered dotted inclusions of pyrite or some flaky inclusions
3. Dendritic inclusions common.
EDXRF analysis
Presence of Si, Fe, and Ca
Presence of Si, Fe, and Ca
FTIR spectroscopy
Absorption band in the region 5350–5000 cm-1; hump ranging from 4600 to 4300 cm-1; detector saturated at wavenumbers below 4000 cm-1
Absorption band in the region 5350–5000 cm-1; hump ranging from 4600 to 4300 cm-1; detector saturated at wavenumbers below 4000 cm-1.

Refractive Index. All the synthetic fire opals gave RI readings in the range of 1.380 to 1.405 (four at 1.380, 11 at 1.390, four displayed a shadow edge at 1.395, three at 1.400, two at 1.405, one at 1.382 and one at 1.385). All the natural fire opals tested for comparison displayed RIs of 1.400 to 1.435. O’Donoghue (1988) stated that the typical RI value of Mexican opal falls in the range of 1.420–1.430, but on rare occasions it may go as low as 1.37while O’Donoghue (2006) gives a different value of 1.460 for Mexican opal and a range of 1.440–1.460 for opal in general. Measuring a refractive index below 1.40 in a material that looks like fire opal should create suspicion.


Specific Gravity. The Mexifire samples had SG values in the range of 1.63–1.77. These values are low for synthetic opal; Smallwood (2003) reported SGs down to 1.74 for a Russian product, while Gunawardene and Mertens (1984) measured an RI of 1.91 for Gilson polymer “Mexican fire opal.” By contrast, some synthetic opals have shown SG values up to 2.27 (e.g., Kyocera: Quinn, 2003). The SGs for the tested natural opals varied from 1.92 to 2.06. Therefore, SG values that are significantly below this range provide an excellent indication that an opal is synthetic. During the SG measurements, the synthetic specimens did not show any signs of porosity; this also was reflected in the quality of polish they displayed.


Polariscope Reaction. All the synthetic samples gave a strong strain pattern with snake-like bands. By contrast, the strain pattern in the natural opals was much weaker and did not exhibit snake-like bands.


Fluorescence. All samples, natural and synthetic, were inert to long- and short-wave UV radiation.


Absorption Features. In both the synthetic and natural samples, no absorption features were seen with the desk-model spectroscope.


Internal Features. Examination of the Mexifire samples revealed the following features:

  1. Turbidity: Most exhibited moderate-to-strong turbidity when illuminated with a fiber-optic light. In darkfield illumination, however, they appeared transparent. The turbidity was somewhat zonal (figure 4, left), as seen in one of the natural samples examined for this study (figure 4, right) and in some other natural fire opals (Choudhary and Khan, 2007).

Figure 4: Turbid zones were observed in almost all the synthetic samples (left; magnified 20x) and in a few of the natural opals (right, magnified 15x). Also note the scattered pinpoints throughout the stone on the left. Fibre-optic illumination

2. Pinpoints: All synthetic samples exhibited scattered pinpoints throughout the stones (again, see figure 4, left). These pinpoints were best visible when the samples were illuminated with fiber optic light however weak effect was still visible under dark-field. Even at higher magnification, the exact nature of these pinpoints could not be resolved. However, similarly scattered “flaky” inclusions have been seen previously in natural opals, while Gübelin and Koivula (2005) mentioned tiny grains of pyrite scattered throughout a stone. We did not find any reports of such “pinpoint” inclusions in a natural opal.

3. Whisker-like Inclusion: One of the synthetic fire opals displayed a whisker-like inclusion (figure 5, left) that broke the surface. Its exact nature could not be determined, but it looked like a hollow tube filled with an epigenetic material. One of the natural opals also displayed a similar inclusion (figure 5, right). Johnson et al. (1996) reported similar-shaped inclusions in fire opal from Ethiopia.

Figure 5: One of the synthetic samples contained the whisker-like inclusion on the left (magnified 75x), which appears to be a hollow tube filled with an epigenetic material. A similar feature was seen in one of the natural fire opals (right, magnified 80x). Fibre-optic illumination.

Over the years, synthetic and imitation opals have been differentiated from natural material by the presence of a “lizard skin” or “chicken wire” effect and/or a columnar growth pattern (e.g., Gübelin and Koivula, 1986, 2005; O’Donoghue, 2006, 2007). Detailed examination of the inclusions and growth patterns in the Mexifire samples was conducted using various types of illumination and techniques, but no “lizard skin” or “chicken wire” effect was observed. This effect is visible only in synthetic opals made of uniform-sized silica spheres with a close-packed structure, which serves as further evidence that the structure of these synthetic opals is different from that of most synthetic or imitation opals described previously. One exception involves some Russian synthetic opals that also do not show a “lizard skin” effect (Smallwood, 2003).


Some additional internal features seen in the natural opals that were studied for comparison are shown in figure 6. See Gübelin E.J., Koivula J.I. (2005) Photoatlas of Inclusions in Gemstones, Vol. 2. Opinio Publishers, Basel, Switzerland for in depth illustrations of inclusions in natural opals.

Figure 6: Some of the natural fire opals studied for comparison displayed internal features such as wavy flow patterns with colour concentrations (left, darkfield, magnified 45x) and a feather-like feature (right, fibre-optic illumination, magnified 80x) that were not seen in their Mexifire synthetic counterparts

EDXRF Analysis. Qualitative EDXRF spectroscopy of all the samples, synthetic as well as natural, revealed the presence of Si as the major element, which is expected for opal. In addition, the samples in both groups contained traces of Fe and Ca. We did not detect any Zr, which has been used for impregnating and stabilizing opal (Webster, 2002; Smallwood, 2003).


FTIR Analysis. The infrared spectra recorded for the synthetic and natural samples displayed similar features in the range 6000–400 cm-1 (e.g., figure 7; also compare to Johnson et al., 1995). Slight differences in the intensity and appearance of the absorption pattern were caused by variations in the amount of transmission; better transmission revealed sharper absorption features. All samples had an absorption band in the region 5350–5000 cm-1; this feature also consisted of sharp peaks, depending on the transmission. A hump was observed in the 4600- 4300 cm-1 range, often with small peaks in all of the synthetic samples and seven of the 12 natural opals in this study. The absence of the absorption feature at 4600-4300 cm-1 may provide a useful identification criterion for determining natural origin. The detector was saturated by strong absorption at wavenumbers below approximately 4000 cm-1.


Some Inamori/Kyocera products are widely known as imitations rather than synthetics because they use polymers as binding agents. Although the low SG values of the Mexifire products are consistent with the presence of polymers, this could not be checked with IR spectroscopy due to complete absorption in that region of the spectrum. Nevertheless, the manufacturer claims that no polymer is present in these Mexifire opals.
















Figure 7: These representative IR spectra of Mexifire synthetic opal (top) and natural fire opal (bottom) exhibited similar features: an absorption band in the 5350–5000 cm-1 region, a hump in the 4600–4300 cm-1 range, and total absorption below approximately 4000 cm-1.

Conclusion

Similarities in some of the gemmological properties, as well as in the chemical composition and IR spectral features, were noted between Mexifire synthetic opals and their natural fire opal counterparts. However, a low SG value (<1.77) is an excellent indication that a fire opal is not natural, and additional evidence is provided by a relatively low RI value (<1.40) and internal features such as scattered pinpoints. In addition, the absence of a hump at 4600 - 4300 cm-1 in the IR spectrum suggests that the sample is natural.


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All photographs and photomicrographs by Gagan Choudhary


Refer Part 2 for updated properties of Mexifire opal