Opal in Indonesia
A LITTLE HISTORY
According to ‘Sujatmiko’ (Australian Gemmologist Volume 22, Number 6) opal was first discovered in Indonesia during geological mapping by Dutch geologists during the 1930’s. However there was no mining development unto the 1970’s. So very little of the early history of opal in Indonesia is known.
According to Tay Thye Sun et al, in an article “Precious opal from Java” in the Australian Gemmologist Volume 23, Number 11 of 2009, The introductions states:
The presence of precious opal was first mentioned in the annual report of the Mining Department (Jaaboek van het Mijiwezen) dated 1936-1937. In the chapter on Geological Investigations, Useful Raw Materials, (p17), an excerpt is given below:
“In the North-eastern part of map nr.13 opalizing effects were repeatedly encountered in pumice tuffs. Precious opal of good quality , particularly the black variety with a good play-of-colours, have until now only been encountered in small amounts near Genteng. It is the first time that this kind of material has been encountered in the Dutch East Indies”
Irwan Holmes writes in “Fireworks at Midnight “ Lapidary Journal, Volume 40, Number 3 of June 1986 that no mining was done before World War 2, and then in the late 1960’s, mining was as a hobby by local farmers. (read more of the story here).
The original opal fields, as mentioned above were mined, more in the later half of the 20th century, and in a region of Banten Province near of “Maja” These perhaps are known as the “original fields”. These fields were extensively mined in the years 1993 - 2000 (Coldham 2020). Today most mining occurs in what may be described as the “newer fields” near the area of Cilayang Village
A more recent article describing “Kalimaya” or Precious opal from Java Indonesia is available in the Australian Gemmologist Volume 27, Number 4.
The Indonesian word Kalimaya pronounced “Ka-li-may-yah” and is translated as Kali (river) maya illusions or “Rivers of illusion” relating to the flowing and movement of colours as the gemstone is turned and moved.
GEOGRAPHY AND MINING FIELDS
From reading the literature regarding opal in Indonesia there now seems to be two separate mining districts of importance. It is possible to distinguish the “original fields” from the “newer fields”.
The original opal fields: Known a the “Maja” fields near Ciburuy Village
According to Sujatmiko et al. The mining area covers an area approximately 13 x 9 kilometres southeast of Rangkasbitung in a slightly hilly area with deeply incised small rivers.
Tay Thye Sun reports the opal is found in volcani- clastic rocks south east of the city in Rangkasbitung, Kabutpaten Lebak in the province of Banten in western Java near the village of Gedong.
The “newer” opal fields, near Cilayang Village
More modern production of precious opal occur in mining fields that occur slightly further in distance south east of Rangkasbitung
A LITTLE GEOLOGY
Old fields geology:- The opals occur in the Genteng formation, a more than 700m thick layer of volcani-clastic sequence of pumiceous tuff, tuffaceous sandstone clay beds, breccia and conglomerates originating from older volcanic arc volcanism on Java. The age of these formations have been estimated as being from the early Neogene Period (Librarians note: The Neogene is the younger portion of what those of us who are a little older knew as the Tertiary period of geology!), and late Pliocene to early Miocene epochs approximately 5-10 million years ago (ma).
According to Sujatmiko, “The opal is confined to a certain pumiceous tuff layer of 0.3m to 2.3m thickness which is fine grained and deeply weathered to a clay rich , soft greenish grey colour. There are abundant, large trunks of fossil wood in the upper portion”.
“Also little is known about the genesis of the opal. A Hydrothermal origin is unlikely (Adiman, 1987). It is most likely that the opal originated from strong weathering of the glassy pumice grains by circulating rain or ground water, which has converted the layer into a montmorillonite clay rich rock and so releasing and re-precipitating this silica as opal.”
Curators note: It would seem that the often quoted report on Indonesian Opal was, probably published as a “Thesis” from an Institution in Bandung Indonesia. Despite many communications with Authors quoting this report, and requests from Bandung, this document appears to have been lost.
My appeal would be if anyone has a copy of the document or knows how it might be obtained please contact me at “International Opal Academy”!
You can read more about the geology of this area here: Watch this space, still working on this content!
LITERATURE
Precious opal from Java - Tay Thye Sun, et al, - Australian Gemmologist - 2009 - Vol23, 11. pp513-528
Fireworks at Midnight - Irwan Holmes - Lapidary Journal - June 1986 - Vol 40-3, pp 20-24.
Black opal from Java - Sujatmiko, et al. Australian Gemmologist - 2005 - Vol22,6. pp254-259.
Indonesian Opal - O’leary and Ball, Australian Gemmologist - 1976 - Vol 12-11, pp327-8
Some observations on the composition and origin if opals from Java - Einfalt, HC. - Journal of Gemmology - 2007 - Vol30.7-8, pp383-398.
A visit to the Indonesian Opal fields in 2019-Opal Types, Mining and Treatments Part 1, Terrance Coldham, Joel Ivey (and with contributions from Anthony Smallwood). Australian Gemmologist - 2020 - Volume 27, No. 4, pp199-213.
TYPES AND VARIETIES OF OPAL FROM INDONESIA
Brown body coloured Precious opal from Indonesia. Technically this material most likely may be classified as “dark opal” in the opal nomenclature with a mention of its brownish body hue. It is possible the material could also be classified as “tea opal” (Irwan Holmes) or Kalimaya Teh, (Coldham) and due to its brown body colour although it is also possible that as the material pictured is more “opaque” in its diaphaneity that strictly speaking “tea opal” should be reserved for more translucent to opaque varieties of Indonesian brownish body coloured precious opal.
Transparent “crystal” precious opal or “Kalimaya Kristal” from the Indonesia.
A newer fields specimen of so called “laser” opal or ‘Ruyung” opal and sample of opalised wood from the “newer” fields in Indonesia
A photo micrograph of the end portion of the sample of “Laser” opal pictured above. Note the emplacement of the precious opal in the tubes or growth structures present from the original biological “tree limb” or branch.
Two samples of so called “limb cast” fossil or opalised wood from the newer Indonesian opal fields and provided to the Academy by Paul Sedawei.
The left hand sample has been”cleaned by preliminary treatment procesess, and teh right hand sample is an example of the “Raw” condition as mined and can be seen covered in the greyish clay material that covers the material as originally mined.
