THE AUSTRALIAN GEMMOLOGIST | Unusual Chalcedony-Agate Pebbles from the Gobi Desert
Unusual Chalcedony-Agate Pebbles from the Gobi Desert
I have been a rock collector for as long as I can remember, from the first time I visited a stony English beach as a child and felt the need to pick up the most interesting pebbles and put them in my pocket.
While my interest broadened over time and now lies in rare and uncommon gems, I felt compelled to purchase a small lot of tiny pebbles from a well-known online auction site recently, mostly to satisfy my curiosity over a gemstone mineral variety which I had not previously encountered. (It helped that they were relatively inexpensive too!)
When they arrived from China and I had a chance to examine the odd little stones first hand, my fascination for these remarkable rocks started to grow. They were described as Gobi Desert Agates, were mostly less than 6cm in width, and came in a variety of different colours and shapes with some other unusual features.
I decided that a bit of research was warranted and discovered that these unique stones originate from the Alashan plateau area in the south-western portion of the Gobi Desert of Mongolia. This is indeed an arid place, as can be sensed in Figure 1.
The agates are reported to be recovered from undisclosed desert locations by nomadic tribespeople, although due to the harsh desert environment, with winds reaching 193kph and temperatures ranging from -38°C at its coldest and +42°C at its warmest in summer, the area is only accessible for a few months each year.
Over time, the desert storms expose the underlying rocks and stones. They are subjected to a natural process of tumbling and sand-blasting, their shapes and outer texture being a result of extreme natural weathering. As a result, no two pebbles are alike; each has its own unique character. A clear example is the little rock shown in Figure 2. It is not an agate, but a (limestone?) rock from the area where the agates are collected. It is approximately 6cm wide and shows the effect that the severe wind has on the geology of the region. The rate of wind ablation is affected by the hardness of the component minerals and the matrix in the rock, resulting in various shapes and patterns, here suggesting a layered structure.
Figure 1. Gobi Desert landscape (photo courtesy of Lan Yao, licence-free image;
https://www.pexels.com/photo/brown-and-gray-mountains-under-gray-clouds-9383559/).
Figure 2. Two views of a wind-sculpted limestone (?) pebble collected from the same area as the chalcedony agates (32.07g; 60x40x24mm).
Figure 3. ‘Eye Agate’ pebbles with multiple ‘eye’ formations. Left: highly weathered multi-coloured pebble (13.57g; 39x28x14mm). Right: rounded pebble, with less obvious yellow-green ‘eyes’ in similar greyish matrix (16.11g; 27x24x19mm).
Figure 4. A collection of eight ‘eye agates’ showing diversity of colour and form (largest, on left, 126g; 56x47x39mm).
Many of the fascinating chalcedony agates are described as ‘eye agates’ because they have concentric, radial, orbicular structures which frequently protrude from the surface of the stones. These structures are often in contrasting colours to the host matrix, displaying a range of different shades. Figure 3 shows two similarly coloured but contrasting specimens. The pebble on the left has a dense field of small greyish-green ‘eyes’, with weathering and the brown rings emphasising the appearance of the ‘eyes’. In contrast, the slightly larger (in weight) pebble on the right, has less obvious yellow-green ‘eyes’ with a body colour in pastel shades.
Figure 4 portrays a collection of eight ‘eye agates’ demonstrating the diversity in colour of both the matrix and the ‘eye’ structures. The largest pebble (126g) on the left, shows several large ‘eye’ structures that are pale yellow with mottled dark brown, set in a frosted blue-grey matrix. On the upper right is a pebble (52.40g) with a rare vibrant yellow body colour and a range of slightly protruding ‘eye’ structures in rings of grey and brown.
An interesting sub-variety of these agates is known as ‘Black Mountain Eye Agate’. These stones are characterised by a steely-blue body colour, heavily veined surfaces, and ridged ‘eye’ formations with yellow outer rings (Figure 5).
Figure 5. ‘Black Mountain’ variety of ‘eye agate’ with grey-blue body colour (22.84g; 37x34x15mm).
Figure 6. Two perspectives of the same translucent ‘eye agate’ pebble with concave yellow ‘eyes’ (104.49g; 45x44x41mm).
Figure 7. Left: translucent, frosted lilac ‘eye agate’ pebble with yellow ‘eye’.
Right: the ‘rear’ view of the same stone in transmitted light showing its translucency (139.48g; 64x60x40mm).
Figure 8. Left: translucent ‘eye agate’ in the shape of a duckling (17.44g; 29x27x21mm). Right: a highly textured ‘eye agate’ pebble with sculpted spur (38.63g; 45x34x29mm).
The two images in Figure 6 display the same good-sized pebble from different angles. In contrast to the specimens in Figures 4 and 5 which are generally opaque, this pebble has superb translucency and unusual dimpled, rather than protruding, ‘eye’ formations. Another example of transparency, and with bigger ‘eye’ structure, is the large stone (139.48g) viewed from different directions in Figure 7. This stone displays a pale lilac body colour with a single vivid yellow ‘eye’ formation. Its exceptional translucency is enhanced when viewed via transmitted light (Figure 7, right). It also displays an interesting wind-sculpted shape, with a protruding ‘eye’.
Along with attractive colours, the more extreme the shape of the stone, the more desirable it becomes to collectors. The odd-shaped pebble with its protruding ‘eye’ in Figure 7 is one example, and Figure 8 shows two further examples of ‘eye agate’ pebbles that demonstrate the variety of colours and shapes determined by the action of the region’s harsh desert winds. The little ‘duckling-shaped’ pebble (Figure 8, left), with its smooth glossy surface is particularly attractive to collectors. The larger pebble (Figure 8, right) is a quite differently textured example of ‘eye agate’. It has a protruding spur making it reminiscent of a whelk shell. In stark contrast, yet again, Figure 9 shows an elongated pebble with a single ‘eye’ surrounded by weathered concentric growth features in delicate pastel shades ranging from yellow to green to lilac.
Figure 9. A weathered pebble with a single large ‘eye’ and attractive pastel shades (19.31g; 29x13x8mm).
Figure 10. Two ‘vein-type’ pebbles. Left: yellow veins stand out against a brownish matrix (9.06g; 30x18x13mm). Right: a complex pattern of veins in different colours (12.91g; 29x27x18mm).
Figure 11. A ‘vein-type’ brown agate with spur. (28.11g; 38x32x25mm).
Another variety of agate from the region displays vein-like, interconnected ridges which stand proud of the surface of the pebble, perhaps the result of wind-vortices playing over the surface of the exposed stones, and different rates of weathering according to variations in hardness. The two smallish, sculptured pebbles in Figure 10 are multicoloured; the pebble in Figure 11 is uniformly brown with a spur, possibly due to differential exposure to harsh winds. Yet others of these remarkable stones display an unusual habit, with masses of botryoidal chalcedony (Figure 12).
Figure 12. Top: two botryoidal pebbles (each approximately 12g; 35x20x16mm). Bottom: three clusters of botryoidal chalcedony with some ‘eye’ structures visible.
The ‘eye agates’ described above are fascinating and variable. But how did they form? Internet searches reveal very little credible information about these stones, but two thoughtful blogs were helpful. One was by Meredith Meow (2022) who described what little evidence she had gathered about ‘eye agates’ from six locations in Mongolia. Included are some images taken from other sites showing ‘eye agate’ material in a matrix which looks like shale or sandstone, clearly indicating that the agates were included in conglomerate-like sedimentary rocks in an already weathered and rounded form.
Meredith Meow (2022) referred to a second blog written in Chinese by He Jing (2019), a geologist who purchased some ‘eye stones’ in Alxa (or Alashan in Chinese) province, located at the western part of Inner Mongolia. He Jing had read a “popular science” article that mentioned “silica bacteria stromatolite fossils from 2 billion years ago.
These fossils are eye stones, which are microbial fossils from 2 billion years ago” [Google translation]. This quote prompted He Jing to prepare and analyse thin sections of the ‘eye agates’ to search for evidence of a biological origin. The blog contains many detailed images and from careful examination at different magnifications using a polarising microscope, He Jing found that:
“…the “eye” is composed of siliceous laminae and ferrocalcite laminae about 0.1-1.0mm thick, which indeed contains thick, about 1.0-1.7mm, microbial lamina with bacterial fiber structure. It seems that the formation of some ‘eye stones’ is really related to microorganisms! The microbial layer in this ‘eye stone’ has a bacterial fiber structure. In the outermost layer of the microbial layer, some filamentous and tubular cyanobacteria (?) distributed vertically in the microbial layer can be seen in clusters.” [Google translation]
The information from this blog indicates the ‘eye’ structures likely have a microbiological origin with replacement of bacterial mats from stromatolites by silica and other minerals, with much of the colour due to ferrous iron. The differential hardness of alternating rings of siliceous and calcareous minerals would explain the concentric pattern of weathering in many of the ‘eye’ structures.
Figure 13. The orange (top left: 23.76g; 41x21x21 mm) and green (bottom: 41.15g; 61x29x18mm) stones from Madagascar have been dyed and possibly carved; the third stone (brown; top right: 38.15g; 37x34x28mm), possibly treated, is from Morocco.
The rounded pebbles are likely to have been ‘recycled’ possibly more than once; that is, they have originally been wind and/or water-worn and/or tumbled, then deposited and cemented in a matrix, like a conglomerate, and are now being weathered out of this rock. There are some YouTube videos (in Chinese) by “Novelty Hunter” showing fractured stones being picked up on what looks like flood plain, so the weathering cycle continues.
After more than a decade of collecting the pebbles from the desert plains, supplies are now dwindling, and prices of these ornamental stones are increasing. On a cautionary note to prospective buyers, some less-than-scrupulous online vendors advertise artificially coloured stones that are often mislabelled as ‘Gobi eye agates’ (Figure 13). The rather luridly coloured orange and green pebbles are polished specimens of dyed Madagascan orbicular jasper, not Gobi ‘eye agates’. Another variety known commercially as ‘Panda-type’ Gobi agate is also shown in Figure 13. These are usually bi-colour grey-brown and white botryoidal agate nodules, natural and quite attractive in themselves, but which I discovered are Moroccan in origin.
For my part, the endless variety and unique beauty of the chalcedony agates from the Gobi Desert means that a handful of pebbles has grown into a shelf full of them! Perhaps those who share this unfortunate habit will understand my obsession, one which over the years has left me a little poorer, but considerably happier.
Photos courtesy of the author (Gemgazer.com) unless otherwise stated.
Acknowledgement: The author would like to thank Léonie Rennie for her assistance with proofreading the manuscript and for her valuable contributions.
References
Jing, H., 2019. I really saw the microbial layer in the ‘eye stone’ I bought in Alxa. He Jing Science Network blog [online] Available at: <http://blog.sciencenet.cn/u/cqhj3> [Accessed 11 September 2023].
Meredith Meow, 2022. Some information I gathered about gobi desert eye agate. [online] Available at: <(https://www.reddit.com/r/rockhounds/comments/v8uf76/some_information_i_gathered_about_gobi_desert_eye/> [Accessed 11 September 2023].
