A Visit to the Indonesian Opal Fields in 2019 – Opal Types, Mining and Treatments Part 2

Terrence Coldham, BA (Geol) FGA FGAA
terry@intogems.com.au Joel Ivey, BSc (Geol)
Anthony Smallwood, FGAA GG (GIA) MSc (UTS)

Abstract

Part 1 of “A Visit to the Indonesian Opal Fields in 2019” provided a description of the rock types and fossiliferous remains associated with Indonesian opal, the mining and processing methods used, and details of a field demonstration of heating opal to improve play of colour. Part 2 provides more detailed information on the physical characteristics and gemmological properties of the opal, fossil plant material and gastropod specimens collected, as well as reporting on heat treatments conducted by the authors. A summary provides further comment on the geological setting, possible sequence of deposition events and opal formation.

Background

During the trip to the Indonesian opal fields, it became apparent that the types of opal obtained from the area visited had changed over time. Earlier publications describe the majority of production as being for example:

• “Crystal, white opal, tea opal, black tea opal and black matrix opal” (O’Leary, 1976);
• “Contra-luz opal, tea coloured opal, chameleon opal, black opal, white milky opal” (Banker, 2001);
• “Transparent to opaque opal showing good play of colour and in colours including brown, yellow, white and black” (Sun et al., 2009).

These articles referred to Indonesian opal as having a tendancy to craze (O’Leary, 1976; Banker, 2001) and often being typical of hydrophane opal (Sun et al., 2009).

Whilst in the field we were informed that past production from locations like the Maja Field (see Map 1, Part 1; Coldham and Ivey, 2020) was characterised by white, yellow, and caramel-coloured absorbent opal locally known as “Kalimaya Bunglon” (local term for chameleon because it changes colour when moistened) or “Kalimaya Kamel”¹. However, during our trip our observations of opal samples suggested there was presently little production of highly absorbent material from the fields.

Examination of Cut and Polished Specimens

Specimens of production early 2000 – absorbency observations

Author JI supplied a parcel of thirteen small cabochon opals of various types (SpNo.JI1) for research purposes which he had collected whilst visiting the opal fields twenty years earlier. In “Precious Opal from Java” (Sun et al., 2009) details are given of experiments to gauge absorbency and we attempted to replicate their absorbency test by immersing JI’s stones in water. Within a few minutes the stones showed signs of change; bubbles appeared on the surface of all but two samples and quite rapid changes in colour were observed. After 10min, some stones had started to bleed brown colour into the water. Other stones had reduced play of colour (POC), several changed to a darker colour and some became more translucent. When removed from the water and allowed to dry, all samples returned to their previous appearance (except for one piece which broke in two). There was little change observed in the pre-and post-experiment appearance of the stones that had bled colour into the water. The two pieces of transparent opal with subdued POC remained unchanged throughout the experiment (Figure 1).

These results seemed to be consistent with statements made by Dain (2020, pers. comm.), the local mine owner who conducted the heat treatment experiments described in Part 1, and others during our visit who stated material produced twenty years ago from the Maja field “stuck to your tongue” and changed colour when wet. We were informed it was mined from wet formations, dried slowly with a lamp and polished whilst dry. It was noted by the authors of that time that after cutting as a cabochon, if the opal got wet it could lose some POC and/or change colour (Sun et al., 2009).

In an attempt to dissolve the brown stain so as to conduct a chemical analysis, we immersed in acetone the cabochon that had previously bled brown colour into water. The results were surprising. After a short time, the stone started turning black, becoming totally black within 60min, and then returning to its original colour when left to dry out (Figure 2). To date,  attempts made to analyse the chemical composition of the brown stain seeping from these samples have been inconclusive.

The reason(s) for the observed change in colour have not yet been determined, however they are consistent with a statement made by Dain when describing a cabochon of “Kalimaya Bunglon”: “So wet, it’s dark coloured. Dried it’s white and the jarong [POC] is more visible and brighter” (Dain, 2020, pers. comm.).

The authors were informed the presence and production of absorbent opal on the fields these days was rare, but it can still fetch high prices if it displays good POC. However, well-proportioned cabochons are rarely produced as most production is now of limb cast material which is usually black such as “Kalimaya Sempur” or in lesser quantities as “Kalimaya Krystal” and “Kalimaya Teh”.

Specimens purchased online

SpNo.PO1a (11.98cts; 37×9.8×4.9mm) and SpNo.PO1b (6.70cts; 18.1×10.1×4.7mm), two pieces of “Kalimaya Sempur Ruyung Laser” displaying fine lines and spots of POC on a dark brown to black surface, were purchased online and have most likely been heated prior to fashioning according to the seller (Dain, 2020, pers. comm.). The shape of each appeared to result from the polishing of a flattened section of a twig or root. Some small amounts of POC appeared in “strings” on the upper surface and as fine pinpoints on ends under normal daylight conditions (Figure 3), however using a strong fibre optic light source increased the visibility of the POC (Figure 4). One polished side showed no POC. 

When we received the specimens, we were surprised to find the POC was not nearly as vibrant as in the video images. The position and shape of the opal displaying POC appears to be determined by the internal structure of the original plant material.

Field samples purchased from miners

SpNo.PO9

A single piece, SpNo.PO9, (25.21cts; 50x12x7mm) was selected from a parcel of seven pieces of limb cast provided to us by Dain for detailed examination. We were informed that none of these specimens had been heated. All the pieces in the parcel were shaped like small twigs or stems, some with sawn and some with broken ends. All were of similar colour, dark brown to black, with the exterior partially covered with a thin, fawn-coloured coating. Some had raised ridges on their exterior surface as if representing leaf nodes. Almost no POC was observed in normal daylight, however in strong light under magnification, very fine pinpoints of POC could be seen on section surfaces (Figure 5). A specific gravity (SG) of 1.76 and a refractive index (RI) of 1.36 were recorded.

The shape of SpNo.PO9 was reminiscent of a twig or stem with an oval section as if it had been flattened. One end was a broken surface and the other a polished flat surface. The exterior surface was dark brown to black and partially covered with a fawn-coloured coating. There were five raised ridges on the exterior running at right angles to the long axis extending from the sides into the centre.

Careful examination of the external surface and one polished end using a 10x hand lens under a normal daylight desk lamp revealed no POC. When examined under magnification (75x) with strong fibre optic lighting some small pinpoints of blue POC could be seen with difficulty (Figure 5). Small blebs of what looked like pyrite were also noted. The external shape and internal structure of this specimen resembled those of plant matter. The small amount of POC observed in the very fine tubes of opal are assumed to have been present when mined and was structurally orientated to the original grain. We were told by Dain such material would react well to heat treatment (Dain, 2020, pers. comm.).

SpNo.AM2

The external features of SpNo.AM2 (52cts; 40x25x15mm), as with SpNo.PO9, were also consistent with a limb-cast, but much thicker in diameter. The specimen was sawn into three pieces and a face polished on one piece. Under low magnification (10x), fine light brown lines on an opaque black background material were observed, perhaps reflecting the original wood grain. A small amount of opaque white material, suspected to be opal infilling a void, and small inclusions of what appeared to be pyrite, were present (Figure 6). A thin section was prepared which under magnification (70x) showed a cellular structure. SEM analysis indicated the major elements present as being Si and O (Figure 7). SG was measured as 1.98.

The observations and analysis were consistent with some “wood like” plant matter being replaced by amorphous opaline silica making the material opalised wood.

SpNo.AM1

This specimen was a small section of a limb cast sawn into three specimens each with polished faces (Figure 8). The concentric growth rings could be seen surrounding a central core made up of many tubes, circular in section, and running the length of the specimen as was a crescent shaped channel filled with clear transparent material displaying a small area of pale blue POC (Figure 9).

Careful microscopic examination revealed a cellular structure with POC opal apparently infilling the cells of the original plant material indicating the specimen was opalised wood (Figures 9 and 10). An SG of 2.01 and an RI of 1.442 were measured.

A thin section of SpNo.AM1b was cut perpendicular to the long axis allowing a clear view of the structure. This sample had properties and analyses consistent with opaline silica, however as POC was observed it appears the specimen could be described as fine tubes of precious opal within an opalised wood host (Figure 10).

An additional thin section of SpNo.AM1b was prepared that clearly revealed the cellular structure of the original plant material. SEM analysis was carried out on two surfaces, one longitudinal and one transverse, indicating in both cases the major elements present were Si and O (Figure 11).

Information on the type of fossil plant matter was sought from fossil plant researcher Dr. Steve McLoughlin who suggested the specimen may be opaline replacements of whole vascular strands in ‘palm’ wood, generally assigned to the fossil genus Palmoxylon (McLoughlin, 2021, pers. comm.). The thin sections appeared similar to thin sections found in fossil Pandanus (Viney, 2008).

SpNo.AM10

A small sawn and polished transverse slice of a piece of plant matter, SpNo.AM10 (1.53cts; 8.4x7x4.5mm), had a fawn-coloured external coating covering a thin layer of black material surrounding a transparent clear core, and also displayed subdued POC. Enclosed in the clear section was what appeared to be vegetable matter in the form of rod shaped, brown, apparently hollow tubes, possibly fine rootlets (Figure 12). RI was measured at 1.435. We interpret the clear section with POC as likely to be precious opal infilling a hollow section of a stem of some sort into which have grown rootlets.

SpNo.D121e

This polished specimen (20.5cts; 21x9mm) is a tubular-shaped piece of transparent, slightly milky opal with included brown root-like structures covered in fine colourless crystals. In a recent publication (Chauviré et al., 2020), the authors describe the occurrence of a fossil cicada exoskeleton preserved in Indonesian opal, deposited on the surface of which were fine transparent crystals which they identified as zeolites showing features of clinoptilolite–heulandite. The crystals on the root-like inclusions in specimen SpNo.D121e look very similar and considering the similar geographic origins of both specimens, it is possible that the crystal growths in SpNo.D121e are the same zeolite and that this is a specimen of plant roots entombed in opal, implying that the opal infilled a void into which fine roots had grown (Figure 13).

SpNo.AM5

Specimen SpNo.AM5 (6.43cts; 20x15x8mm) is milky white opal surrounded by brown material identified by author JI as decomposed tuff with the appearance of opal enclosing dark brown rod-shaped inclusions. The conchoidal fracture surface is glassy. The specimen appears to be made up of several layers terminating in a very flat textured surface. Under low magnification (10x) the layering is more obvious and is parallel with the top flat surface (Figure 14, left) which appears as a very thin dense white layer with a finely textured surface (Figure 14, right). Enclosed in the white opal were pieces of what appeared to be carbonaceous small stems or roots. The specimen had characteristics of being the broken half of an opal-filled cavity that, prior to infilling, contained a few pieces of plant matter. One small piece of plant matter appeared to be “floating” on the surface (Figure 14, left).

SpNo.PM6

Twenty-one gastropod (snail) specimens (total weight 210cts) were purchased at the mine from where they originated (see Figure 13, Part 1; Coldham and Ivey, 2020). Some were complete forms and others only partial. The specimens were identified as steinkerns i.e. opal infillings of moulds created by snail shells buried at the time of sedimentation (E. Smith, 2020, pers. comm.).

Three were selected to become sample SpNo.PM6. Two of the samples had a shape similar to a specimen acquired by the Australian Opal Centre, Lightning Ridge, NSW (see Figure 3, Part 1; Coldham and Ivey, 2020). All three snail casts appeared to be composed of translucent to opaque opal varying in colour from greyish-white to dark brown. No POC was observed. Most had exterior surfaces coated to some extent with what appeared to be mud or fine sediment (Figure 15).

Other steinkerns (SpNo.PM7) displayed layers of opal of varying colours and flat surfaces, possibly menisci parallel to these layers (Figure 16). The layering and flat surfaces seemed to reflect ellipsoidal infilling of the original cavity by successive deposits of opal, which in some cases was incomplete. Further information on these specimens is being sought from palaeo malacologists.

SpNo.PM3

Specimen SpNo.PM3a is a rounded elongated lump, (1100 cts; 70x60x50mm) with a partially polished top, composed of hardened fine sediments, carbonised plant matter and opal displaying very small amounts of POC. On the reverse side is a small area of green translucent material that reacted to LW UV light with strong blue-green fluorescence. The top rounded section appeared to reflect the shape of a void created by the decomposition of a limb cast possibly filled with a mixture of clay, carbonaceous material and precious opal (Figure 17).

The green translucent material at the back of the specimen gave off a faint pine resin odour with a hot point. SEM analysis indicated the major element present was carbon (Figure 18). All the properties were consistent with the green material being either amber or copal resin, perhaps remaining in the original plant matter after decomposition.

Heat Treatments

Field heat-treated specimen

SpNo.PM4d (PM4 in Part 1)

This specimen is the end section of the limb cast (460cts; 130x40x30mm) used in the field heating demonstration previously described (Part 1; Coldham and Ivey, 2020). It was re-examined twelve months after the field heating demonstration, providing a chance to examine any changes occurring in the sample. It was observed that the unheated specimen included some translucent opal with greenish to greyish body colour displaying subdued POC. After treatment, the opal displayed a more intense blue-green POC in a now white body colour, especially when wet. After twelve months the POC in the opal was subdued when dry however when the surface was moistened (with water) it appeared very similar to its appearance at the conclusion of the heating demonstration, indicating the changes brought about by heating had persisted for 12 months and were possibly permanent (Figure 19).

Heat treatments conducted by the authors

The results of the heat treatment field demonstration described in Part 1 (Coldham and Ivey, 2020) were so intriguing the authors decided to try and replicate them. Several specimens obtained from the Indonesian fields were heated using two different methods:

• Wrapping specimens in paper-lined foil and placing in the coals of a fire.
• Frying the specimens in vegetable oil.

The results were as surprising as the results we observed in the field demonstration. In every case the POC in the specimens increased markedly in intensity, and most areas of opal became white and less transparent.

The increase in the intensity of the POC did not appear to be due to dark or black material forming or being deposited around or throughout the opal, as one would observe in treated matrix opal or dyed or “smoked” hydrophane opal.

Ongoing research and analysis is planned to help understand the processes involved in the changes to observable POC in these opal samples brought about through heating. 

Heating in paper-lined foil in coals

SpNo.PM5

A piece of partially polished non-absorbent translucent light opal originating from one of the pieces pictured in Figure 29, Part 1 (Coldham and Ivey, 2020), locally termed “Kalimaya Cai Kalapa”, was chosen for a heat treatment experiment.

This specimen (33.32cts; 27x21x19mm) consisted of mostly solid translucent to transparent material with small amounts of fawn, clay-like sediment enclosed and attached on the margins. Some areas of mostly blue POC were observed interspersed within the fawn material along the edges of the specimen. Using a strong torch beam some faint POC was observed internally (Figure 20).

To obtain details of the gemmological properties of this material two small cabochons were cut from material originally adjoining this piece. They displayed very little POC under strong lighting, had an SG of 2.02 and RI of 1.40 and when immersed in water for over one hour showed no sign of absorbency.

The specimen was wrapped in paper-lined foil from a cigarette packet and heated in the coals of a fire for 10min. After heating and being allowed to cool to room temperature, the bulk of the sample became opaque white with evidence of fine fracturing, perhaps due to heat. Some bright blue POC was present in places where there had been subdued POC evident in the unheated specimen. 

A very thin black coating covered the bulk of the stone but did not penetrate the opal. The areas of fawn sediment became completely black and the POC amongst it was more pronounced. This appeared to be due, in part, to the small sections of opal now being surrounded by black background, however this did not seem to entirely account for the stronger POC developed after heating (Figures 21 and 22).

SpNo.AM9

This specimen is a small limb cast (7.72cts; 15.3x9x7.5mm) with a sanded surface partially covered with a fawn coating and was believed not to have been heated previously. This specimen was sawn longitudinally resulting in two specimens, SpNo.AM9a and SpNo.AM9b.

Three faces, one longitudinal sawn face and both end section faces were polished on SpNo.AM9a (3.84cts; 15.5×7.8x5mm). The larger longitudinal surface was dark brown to black in colour displaying an internal and external structure that one would expect in a twig. At one end an amount of translucent opal without POC was observed under a strong fibre optic light source. It appeared to be filling a void in the centre of the limb cast. The internal lining of this infilling showed very small spherical black material. The specimen was fried in vegetable oil resulting in distinct changes to the opal infilling the centre of the specimen, in particular the development of POC where there was very little or none before (Figure 26).

SpNo.AM9b

Specimen SpNo.AM9b (3.69cts; 15.5×7.5×5.6mm) was polished on the transverse and longitudinal faces. The material was dark brown to black in colour and displayed an external structure one would expect in a twig. One end had a centrally located void infilled with translucent opal displaying POC when viewed under strong fibre optic lighting. The stone was heated in vegetable oil for a few minutes.

When observed under desk lighting before heating the infilling of opal on both surfaces appeared to be transparent grey. The end section was dark grey to black displaying a structure expected in a transverse sectional view of a twig. One area had polygonal patches of cream-grey coloured, translucent material. No POC was observed. After heating, some dramatic changes occurred. The core infilling, when viewed on both longitudinal and transverse surfaces, changed to translucent white crazed opal and displaying minor pale blue POC. Strong streaks of POC appeared throughout the larger transverse polished face seemingly controlled by the original wood structure. The transverse section displayed strong changes with bright, multi-coloured POC appearing in an area towards the centre of the original twig. Interestingly, the patches of translucent cream-grey material observed before heating turned opaque black. The distribution of POC appeared to be controlled by the original wood grain (Figure 27).

To check if this POC was not just a surface feature a 5mm slice was sawn off the end that showed the POC. On the resultant sawn surface of SpNo.AM9c, POC was clearly visible suggesting the POC extended throughout the specimen and was not just confined to the surface (Figure 28).

Discussion and Conclusions

Summary

Unlike Australian opals found in sedimentary rocks in marine and freshwater deposits around the Great Australian Basin, the precious opal found in Banten Province in Indonesia is hosted in volcanic rocks. Mapped as the Genteng Formation (Sun et al., 2009) we witnessed opals being dug from a sequence of andesitic to dacitic volcaniclastics and flows consisting of crystal, lithic and pumaceous tuffs with colours ranging from beige and tan to forest green. Opal-bearing beds occurred in a near horizontal to undulating layer most likely draped over the topography. In some opal beds, the tuffs were ubiquitously opalized and contained disseminations and vugh linings of tiny glittery quartz crystals.

On the mine dumps were found leaf impressions, carbonised material with the external and internal characteristics of branches, twigs, stems, and roots embedded in the green and tan coloured tuffs in which opal is normally found. These fossils, along with opal infilled casts of what appear to be land snails indicate the surface areas between the deposits of successive layers of tuffaceous material was well vegetated. This is further supported by the presence of amber found throughout the area as reported to us (Dain, 2019, pers. comm.). We were told that when heating specimens of limb cast material such as SpNo.PM4d in Figure 19, there is often a pleasant resinous odour given off (Dain, 2019, pers. comm.).

We visited a home gallery on the outskirts of Rangkasbitung with exhibits of polished petrified woods and jaspers from the Banten area. There were several ton-sized boulders composed of masses of leaf imprints and limb casts in a groundmass of chalcedony and volcanic ash from other layers in the same volcanic formation created by less violent ash eruptions. 

The proximity of eruptive centres in the area left behind a great deal of volcanic deposits representing from low to high energy events. The district is known for its petrified forests defining the period of sub-aerial volcanism. 

Author JI suggests that it appears the layers of fine ash and interbedded coarser dacite crystal-lithic tuffs which compose the Genteng Formation, are somewhat predictable as the miners sink their shafts. The more violent deposition of the lithic tuffs has knocked down forests and buried the paleo-landscape in thicker layers were highly porous and permeable. Oxygenated surface waters easily penetrated the volcanic debris seeping through the pile, leaching silica and other minerals. These are the layers which host the petrified logs. These beds locally lie upon layers of finer less porous and permeable ash. There appears to have been entrapment or ponding of silica saturated fluids perhaps from the decay of the siliceous tuffs above the finer ash layers, creating the petrified woods, casts and the infill and coatings of opal found scattered in the Banten region. The fact that the leaf impressions did not appear to be burnt when buried by the ash suggests that it could not have been excessively hot. The buried plant matter must have either been replaced by opaline silica or just rotted away creating voids to become limb casts filled with sediment or opal. 

The opal seems to be mainly associated with the remains, or evidence of the remains, of vegetation or organisms such as snails. It commonly occurs as solid pieces of pure opal (see Figures 18, 21, 22, 26 Part 1; Coldham and Ivey, 2020), in lumps mixed with clay and carbonaceous material (see Figures 15, 29 in Part 1; Coldham and Ivey, 2020; and Figure 17, Part 2), as fine veins, patches and layers associated with carbonaceous material (see Figures 25, 31, Part 1; Coldham and Ivey 2020; and Figures 3, 4, 10, 23, 24, 26 and 27, Part 2). It can also occur as angular lumps in the tuff (see Figures 7 and 24, Part 1; Coldham and Ivey, 2020) and in shell casts (see Figures 3 and 13, Part 1; Coldham and Ivey 2020).

It has been postulated that Australian opal results from silica rich fluids or gels being deposited in cavities and voids. Smallwood mentions observing “flow” structures and “menisci” in solid opal (Smallwood, 2014).

Many of the Indonesian opal specimens we examined exhibited similar characteristics. The opal in Figures 15 and 22, Part 1 (Coldham and Ivey, 2020) and Figures 14 and 16, Part 2, showed clear banding or layers consistent with being filled with successive phases of fluid creating layers of different coloured opal and patterns of POC. We asked the miners if these layers were orientated in any way when found and were told it is not common to find such layers, but when found they were generally parallel to the surface.

Several of the opalized shell fossils (eg, Figure 13, Part 1; Coldham and Ivey, 2020) displayed layering of the white opal they contained as does the cavity infill in SpNo.AM5 (Figure 14). Both these samples feature a meniscus consistent with those one would expect in a cavity partially filled with liquid. In specimen SpNo.AM5 a very small piece of what appeared to be a section of a carbonised twig or root can be seen as if “floating” on the opal infill.

In other specimens, the original structure of the plant matter appears to have been preserved by being replaced by dark brown to black material which in some cases tested as being opaline silica. These specimens of wood replacement opal often contained precious opal occurring as fine tubes, channels and flat surfaces. When examined in a thin section it was observed that in some cases the opal occurs within what were apparently voids in the original vegetation (Figure 10). Perhaps the opal-forming fluids were originally drawn into the long thin voids observed in “laser” opal by capillary action.

In other specimens the opal seemed to be present in areas where there may have been bark and/or fissures present in the original specimen perhaps due to decomposition of the original plant material. This suggests that during decomposition vegetation becomes desiccated creating shrinkage cracks, sometimes as a rim around a solid core, perhaps being a result of there being an original bark layer (Figures 15 and 28, Part 1; Coldham and Ivey, 2020) and sometimes as irregular planes within the specimen perhaps resulting from shrinkage cracks. These voids would then be infilled with opal. Such a process may explain the veins of opal observed in samples such as the demonstration heating sample (Figure 19).

Considerations on the Formation of Indonesian Opal

The Indonesian fields produce a wide variety of opal types covering a considerable range of properties and descriptions (Table 1). These include opal with and without POC which may be found as either non-absorbent material or absorbent material (hydrophane). Notable examples we observed include:

“Kalimaya Bunglon”. Chameleon opal that darkened in colour and released a brown “dye” when immersed in water and then reverted to its original colour upon drying (Figure 2). The reasons for these changes have not yet been identified and will be the subject of ongoing study by the authors.

“Kalimaya Sempur”. One type of limb cast opal where opal with POC occurs encased in black opalised wood structurally controlled by the original growth structure of the vegetation (Figure 3). It is suggested that the black, non-precious opal found associated with volcaniclastic sediments from Honduras may have formed by the infiltration of silica-rich and porphyrin-bearing fluids derived from adjacent carbonaceous sediments; porphyrins are suggested to render the black colour of the opal-bearing whole rock (Banerjee and Wenzel, 1999). This is a possible explanation for the black body colour in some Indonesian opals; the origin of carbon could be either organic matter from the sediment layers in the Genteng Formation above the opal-bearing horizon or from more recent decomposed plant roots which penetrated the tuff layer (Einfalt, 2007).

“Kalimaya Kristal Teh”. Solid precious opal occurring in elongated “finger shaped” pieces reminiscent of parts of tree limbs or roots (Figure 21, Part 1; Coldham and Ivey, 2020).

“Kalimaya Porcelain”. Large limb casts of solid white opal appearing as fossil branches (Figure 28, Part 1; Coldham and Ivey, 2020).

“Kalimaya Cai Kalapa”. Translucent to transparent solid opal, with or without POC, encasing the remains of what appeared to be plant roots (Figure 13).

The various forms in which opal is found on the Indonesian fields all suggest an intimate association with organic matter in its deposition and/or formation. It is interesting to compare the features of SpNo.D121e (Figure 12) with Chauviré’s description of an insect’s exoskeleton encased in opal from the Genteng Formation where the exoskeleton of the insect was primarily zeolitized with a thin layer showing features of clinoptilolite–heulandite during the alteration of the host rocks and later sealed in opal deposited by silica-rich fluids derived from the weathering of the volcanic host rocks (Chauviré 2020).

This indicates the opal in SpNo.D121e and SpNo.AM10 may have infilled a void into which the rootlets had previously grown, either when the void was empty or when already filled with fluids that later became opal, suggesting the formation of the opal is younger that the original vegetation. The presence of the rootlets in an apparently undisturbed state suggests they have undergone little or no trauma since growth. This would be consistent with the opal being formed at temperatures of less than 55°C as indicated by the presence of the zeolite clinoptilolite encased in opal (Hay, 1981; Einfalt, 2007).

If the small brown piece of material positioned on the top of the opal meniscus apparently infilling the cavity in specimen SpNo.AM5 (Figure 14) is in fact a small piece of fossil twig floating on the surface of the fluid originally filling the cavity then this would suggest the plant material was present prior to opal formation.

Acknowledgements

Peter Blythe: Mineshaft Pty Ltd. Preparation of specimens.

Rod Hungerford: Technical Coordinator UTS. Preparation of thin sections.

Dr Steve McLoughlin: Department of Palaeobiology Swedish Museum of Natural History. Information on fossil plants.

Dr Richard Wuhrer: Centralised Research Facilities, Western Sydney University. Preparation of SEM results.

References

Banker, T., 2001. Getting Indo Opal. Colored Stone, 14(1), pp.512-513.

Banerjee, A. and Wenzel, Th., 1999. Black opal from Honduras. Eur. J. Mineral., 11, 401-408.

Coldham, T.C. and Ivey J., 2020. A Visit to the Indonesian Opal Fields in 2019 – Opal Types, Mining and Treatments Part 1. The Australian Gemmologist, 27(4), pp. 199-213.

Chauviré, B., Houadria, M., Donini, A., Berger, B.T. Rondeau, B., Kritsky, G. and Lhuissier, P., 2020. Arthropod entombment in weathering‑formed opal: new horizons for recording life in rocks. Nature Scientific Reports 10(1), article no. 10575 [online] Available at <https://doi.org/10.1038/s41598-020-67412-9> [Accessed 20 July 2021].

Einfalt, H.C., 2007. Some observations on the composition and origin of opals from Java. The Journal of Gemmology, 30(7-8), pp. 383-398.

Hay, R.L., and Sheppard, R.A., 1981. Zeolites in open hydrological systems. Washington D.C. Min. Soc. America, Reviews in Mineralogy, 4. Mineralogy and geology of natural zeolites, pp. 93-102.

O’Leary, B. and Ball, R.A., 1976. Indonesian Opal. The Australian Gemmologist, 12(11), pp. 327-329.

Viney, M., 2008. The virtual petrified wood museum. [online] Available at < http://www.petrifiedwoodmuseum.org/OligoLouIntroduction.html> [Accessed 20 July 2021].

Smallwood, A.S., 2014. The unique attributes of Australian precious opal. The Australian Gemmologist, 25(6-7), pp. 207-230.

Sun, T.T., Mok, P.C., Paul, S.L., Paramita, M., Arps, C.E.S., Atichat, W., Fritsch, E., Kang, W.W. and Wijaya, K., 2009. Precious opal from Java. The Australian Gemmologist, 23(11), pp. 513-528.

Footnotes

¹ See Table 1, Part 1 (Coldham and Ivey, 2020) for commonly used local terms for the description of opal variants and play of colour patterns, and English language equivalents.