I have seen the rough and cut specimens. The cut specimens are attractive. I also know that a famous temple in Thailand is buying the stones for making amulets and carvings. This goldstone is natural, but there is also man-made glass called goldstone which is a simulant/substitute for sunstone.
Dr Robert R Coenraads (Nikiticorp Australia Pty Ltd) writes:
Summary
Samples of a gem material called ‘Goldstone’ provided to Dr Coenraads by Nikiticorp for gemological testing have proved to be amphibolite, a rare metamorphic rock consisting almost entirely of the mineral amphibole. The amphibole mineral has now been largely altered to hematite, goethite and quartz and it displays a magnificent golden iridescence. This rare material is known only from two other localities in the world, Greenland and Wyoming, USA. Trial lapidary work carried out on this material shows it to be hard (approximately 6 of Moh’s scale) and capable of taking a high polish. The golden iridescence is best displayed at certain orientations so some care must be taken when cutting the rough material.
Introduction
Several kilograms of rough material being called ‘goldstone’ were provided to Dr Robert Coenraads by Nikiticorp Pty Ltd for examination. The material was sliced using a slabbing saw at the Wingala Lapidary Association in Sydney. The rough was sawn in three perpendicular directions, polished and photographed. This was to look for any observable difference in appearance of the material at different orientations. Cabochons were also prepared from some of the slices. A sample was provided to Mr Rad Flossman of the University of New South Wales in Sydney to prepare a microscope thin section for petrological study. Another sample was given to Dr Peter Williams for X-ray diffraction analysis at University of Western Sydney.
Description of the samples
The rough material is not particularly attractive being dull yellow or rusty red brown and powdery in appearance. The rough pieces provided were plate-like, that is larger in two dimensions than the third. When polished, however, the samples show a brilliant and unexpected golden iridescence. Noting the orientation of the cuts with respect to the shape of the material revealed that most beautiful iridescence was obtained when the sample was cut in one of the directions perpendicular to the large face of the rough material. In this direction almost all the grains will appear golden in polished surfaces. In the second direction at right angles and perpendicular to the large face, and in the third direction parallel to the large face of the rough most of the grains appear dark brown, although some show the golden iridescence.
X-ray diffraction analysis
The XRD work shows that the samples from Port Hedland are quite similar to those from Wyoming in that the original amphiboles have been largely altered to the iron oxide minerals, hematite and goethite, and quartz as a result of some form of secondary alteration process. This alteration is probably responsible for the iridescent color being so rich and golden. The XRF pattern also showed that traces of the original amphibole remain within the rock and identified them as either grunerite, manganogrunerite (dannemorite) or cummingtonite.
The thin section also reveals that the original amphibole crystals are oriented, probably as a result of directed pressure causing them to grow in that alignment during their metamorphic formation. It also appears that in some of the samples, the layers that were almost entirely made up of amphibole are interlayered with thin layers of almost pure quartz.
Gemological testing
The four ‘goldstone’ cabochons were tested at the Gemological Association of Australia laboratory. The material was found to be inert under LW and SW ultraviolet light, to have an indeterminate spot refractive index around 1.5 to 1.6, and a specific gravity around 2.60 to 2.80. Being a rock rather than a single mineral the tests for specific gravity and refractive index were considered not to be of use in identification of this gemstone. The binocular microscope and hand lens provide the most positive form of identification; that is this gem stunning visual appearance of the interlocking grains and distinctive golden iridescent sheen.
Previous studies
Amphibole is known from only two other localities in the world:
1. A gem quality iridescent orthoamphibole found near Nuuk, the capital of Greenland.
2. A gem quality iridescent orthoamphibole found near Douglas, in Converse County, Wyoming, USA.
The orthoamphiboles from Greenland were shown by Appel and Jensen (1987) to be solid solutions between the end members anthophyllite and gerdite. The iridescence colors are green, blue, through yellow to gold, red and violet (rare), and are caused by diffraction of light from lamellae of amphibole less than 0.2 um thick. The material has a hardness of 6 and an SG between 3.18 and 3.37. Refractive index is 1.64 to 1.66, with a birefringence of 0.02.
The material from Wyoming is different in that it now consists almost entirely of goethite and/or opaline silica which appears to be derived from weathering of the original ferroanthophyllite. The iridescence of the Wyoming material is mainly golden or dark brown with some red or silver gray.
Comparisons between the polished Western Australian ‘goldstone’ and photographs of polished samples from Greenland and USA suggest that the Western Australian material has a larger percentage of its surface displaying the golden iridescence and therefore it is a more attractive material.
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