By Tay Thye Sun(1), Puah Chun Mok(2), Sunny Liew Paul(2), Mahardi Paramita(3), Charles E.S. Arps(4), Wilawan Atichat(5), We Weng Kang(6), Kumala Wijaya(6).

Abstract

Precious Javanese opal or Kalimaya, as it is called by the local people, a so-called volcanic opal (Fig. 1), has been mined (small-scale) in Central Banten, West Java, Indonesia, since the early seventies.

In January 2006, four authors, TTS,MP, WWK and KW, (Tay Thye Sun, Mahardi Paramita, We Weng Kang and Kumala Wijaya) visited eight (8) opal mining pits near Cilayang village, Curuk Bitung District, Lebak Regency (County) (between 106° to 106° 50 East and 6° 50’ to 7° 0’South). At Cilayang, due to the rainy season, only one pit was presently in operation (Fig.2). Cilayang produces white, brown, dark brown and jelly opal. Black opal is mined in other nearby gem-pits.

Rangkasbitung. Opal occurs as secondary products in specific levels of strongly weathered pumice tuff in the volcanoclastic Genteng formation, part of a succession of tuffs, tuffaceous sandstones, shales, breccias and conglomerates of Upper Tertiary age (coastal, intermontane and intervolcanic basins, Upper Miocene to Early Pliocene; Sujatmiko, et al., 2005) in the western extremities of the Bogor zone, an anticlinal volcanic ridge.

Thirty (30) opal samples were examined using basic gemmological methods. The data conformed with earlier results, that is the R.I. range from 1.43 to 1.45, S.G. 1.98 to 2.02 and displaying under LWUV an inert to weak chalky blue fluorescence with some tinted weak orange at the edge, and generally inert to some very faint blue fluorescence under SWUV. Rough Javanese opal were found embedded between clay minerals, some appeared tube like with rough surface showing rounded imprint from the decomposed pumice rock. Inclusions of Javanese opal tends to have tube-like inclusions with some cavities filled with clay mineral. White patches of silica-like and whitish fissures were found in the opal. Smoky-like brown veins are also present. Some severe fissures caused the opal to break up easily and as a result the local miners would apply layers of polymer over the opal to prevent further crazing. To enhance the blue colour of opal, blue ink were applied to the back of the opal making it more bluish.

Seven samples of white and yellow opal were examined using XRD, and found the composition to be tridymite and cristobalite which can be called opal CT (Cristobalite + Tridymite).

A detailed study of the micro- and nano-structures of the opals was carried out using Scanning Electron Microscope (SEM) and Atomic Force Microscopy (AFM) (Binnig et al, 1986) respectively. SEM shows the typical features of the spherical structure much similar to opal structures from Australia and Mexico (Fritsch et al, 2002). Under AFM examination, magnification under 1nm, the surface of the opal found to have nano-structures almost like some pattern of a carpet with visible individual knots whilst some of the nano-structures were well order, the rest were less orderly. However, it remains to be seen whether the intensity of play of colours is under the influence of the surface texture of the nano-structures.