Indonesian Opal - A Field Report By Paul Sedawie

I visited a new opal mine in Banten which is out from Jakarta, Indonesia. Opal was discovered here in 1995 and there have been various levels of production since then.

There are three main areas where the new Opal can be found.

• 1.Cikole - Maja 
• 2.Rangkas - Bitung 
• 3.Cikole

The Cikole area has a lot of wood fossil. At present there may be thirty crews who are working and they do seem to move around and not make long drives. The first mine I visited was a shallow mine of only 12 meters. It is in a high rainfall area so most of the mining is carried out in the dry season which is from July onwards. The top sides of the mine were supported with bamboo which didn’t look strong to me and the cane ladder was not wide enough for one of my feet.

The walls were saturated with water and there was nearly a stream running at the base of the shaft. I was then told that 12 miners died last year due to collapsing walls. There is no propping or supporting the walls which are very moist even deep inside the mine. The soil is a heavy, sticky kaolinite clay with a grey to brown appearance with black like charcoal spots all through it. They follow the slides in the ground which tend to form opal.

There seems to be an abundance of wood fossil opal which is coal black in colour. Some natural opal rough did dry out quicker after wetting them which is quite unusual because I am used to working with Lightning Ridge material. The wood fossil seemed stable but the crystals were very hydrophane. The material seems brittle like yowah stones and it is softer to cut compared to Ridge material. There is a jewellery market in Jakarta where miners can sell their production.

Observed Opal Treatments

There are large slabs of black potch which are displayed but they are soaked in glycerine. Even large black rubs are soaked in it. It seems that this is a normal practise to soak most material. There are crude jewellery pieces made with base metals and these too have been soaked in baby oil. Even good stones have baby oil on them and I put this down to the fact that the finish isn’t commercial so the oil hides any pits, scratches etc. There is a heating treatment for wood fossils in which they wrap the specimen in foil and heat it with oil and sand and mix it up. The success rate is not very high but they do get a nice surprise with some material. Some of the potch has the clay strongly attached to it and the heat does remove it.

At present I feel that the oil brings the opal colour to life and resin is added so it keeps its polish. The cutting skills of the locals isn’t good and I feel they may use to oil and resin to get a finish. I will be testing some wood fossils in a vibrating tumbler to get a better permanent finish. Another treatment is called natural heat. This is when a stone is placed in water and left in the sun with nothing else for a week.

They say it improves the darkness of the stone but I have cut some in two and can’t see any difference inside. There are some stunning opalized wood fossils making great collector pieces. Some of the opalized wood makes great stones. This opal likes strong in sunlight and can look stunning but in dim light there is very little colour play. Stones with red in them are quite common but most will be distant and a bit leady.

Conculsions

Still a great experience for a hobby cutter to cut a red on black stone. I did see one very striking stone 7 years old that was red and impressive but I didn’t see it in low light. Its base was coal black.

Most of the rough I saw would be classified as N1 to N3 but the brightness level in normal lightning is distant /dull. There is definitely a market for this Opal. I suspect some may be hydrophane and some rough dries out quickly. The crystals look interesting but I only saw a few crystal stones which were reported to be a few years old and stable.

I have sent samples to a renowned scientist who specializes in opal to test these sample and will add to this report. Stay tune.

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Indonesian Opal