THE AUSTRALIAN GEMMOLOGIST | Gemstone Occurrences and Ancient Circular Impact-Scars: the Gemmology Tail Wags the Geology Dog
Gemstone Occurrences and Ancient Circular Impact-Scars: the Gemmology Tail Wags the Geology Dog
Abstract
Numerous occurrences of transparent gemstones are in diverse ways associated with large circular scar-patterns on the Earth’s surface. These are interpreted as vestiges of the asteroid and meteorite bombardment that also scarred the Moon. Neither the existence of these circular scars nor their identification with 4 billion-year-old impacts can be accommodated by presently held versions of plate tectonic theory.

Figure 1. Map showing the circular outline of the Slave Province, slightly elongated in the north-south direction as an artefact of the map projection. Blue circles in the right-hand map represent kimberlites and the blue square indicates the position of the Acasta Gneiss. The base map was compiled by P.B. King (1969), Tectonic Map of North America, 1:5,000,000, USGS. The guide map, in which additional features with circular curvature can be seen, is from J.O. Wheeler et al. (1996) Geological Map of Canada, 1:5,000,000, Geological Survey of Canada.
Introduction
Numerous occurrences of hard transparent gemstones are associated with large circular patterns visible on maps of various sorts and on images obtained through remote sensing. These 2-D patterns are assumed to be surface expressions of deep 3-D geological structures, remnants of the bombardment of asteroids and meteorites c.4100 – 3800 million years ago (Ma) that also scarred the Moon, our neighbour in space. Direct survival of the original impact-scars throughout geological time seems impossible, but constant or sporadic upward regeneration of fractures from below the brittle-ductile boundary may account for the present-day visibility of the circular patterns. Many instances are faint or incomplete, as are the rims of many lunar impact-scars themselves, for which it may be necessary to ‘back away’ from maps and globes in order to see “gross features … missed in the usual quest for more detail” (Hartmann, 2010).
Prevailing geological conventions concerning plate tectonics do not allow for the survival of such scars. But facts are stubborn things and the observed association of gem occurrences with large circular scars is valid whatever may be asserted or denied concerning the origin, survival, or nature of the scars.
The relationships – both geometric and geological – of particular occurrences of transparent gems, within circular patterns, are strikingly different from one case to another. Instances will have to be worked out on a case-by-case basis, as was done for the association of the tanzanite deposit at Merelani with circular scars in East Africa (Saul, 2022). The present paper emphasizes observation, not theory.

Figure 2. Map of New York State showing the circumferences of two circular structures in the Adirondack Mountains. The circles have been outlined on the main map on which a red point indicates the Gore Mountain garnet mines. The Hudson River Valley and Lake Champlain are tangent to the larger, ~160km diameter circle. Geological Map of New York State, 1:1,000,000 (1989).

Figure 3. Geological map of Europe from Keith Johnston’s Physical Atlas (1856?). The Bohemian Massif and continental Europe itself both have circular outlines, as does the Middle-Urals Ring Structure (MURS), discussed below. The Italian peninsula and the Adriatic Sea are situated in positions that would be occupied by an outer rim and an intervening moat to the “European Circle”.
Observations
Canada – diamonds
The 480km diameter Slave Province (Figure 1) is an Archaean Craton located in the northwest Canadian Shield. The province is known for its numerous diamond-bearing kimberlites, and is host for the oldest known exposed rocks, the Acasta Gneiss, metamorphosed 4031–3580 Ma.
Adirondack Mountains, New York State – almandine garnets
The Gore Mountain mines in the Adirondack Mountains of upper New York State host the world’s largest garnet deposit. The mines (indicated by a red dot in Figure 2), are operated for abrasive grade almandine garnet but small gem-quality fragments are not rare. The occurrence of occasional gems within the uplifted Adirondacks may not be exceptional but the neatly circular ~160km diameter core of the Adirondacks awaits an explanation.
Bohemia – pyrope garnets
Chrome-pyrope garnets have been recovered from several localities within the 260km diameter circle of the Bohemian Massif (Figure 3), and used in jewellery since the 7th Century or earlier. Their red colour is attributable to a Cr2O3 content, reported as 1.65-1.75%. Almandine garnets from India and Sri Lanka have long been sold as Bohemian garnets as has pyrope-almandine from Tanzania and Mozambique in recent decades.
Most Bohemian garnets have been recovered from gravels formed by the weathering of Precambrian serpentinized peridotites formed at depths exceeding 60km. Fragments of these rocks were brought to the surface during violent volatile-driven eruptions approximately 25 Ma, with the gems released during subsequent weathering. The massif through which the eruptions occurred is commonly dated >300 Ma.
The Bohemian Massif lies on the circumference of the “European Circle”, which extends east all the way to the Ural Mountains. Its 3200km diameter should not be seen as exceptional as the largest expected terrestrial impact-scar would have a diameter of approximately 5000km, as twice independently calculated from the lunar cratering record (Ryder et al., 2000; Thomas Gold, personal communication, 1986).

Figure 4. Map showing the gem localities in the Urals; yellow circles indicate occurrences of transparent coloured gemstones formed during the collision of Europe with Asia. All or most of the gem occurrences shown here are primary (hardrock). The largest blue area represents the Romashkino Supergiant oil field; the second largest, situated astride the tangent common to the two circles, is the Arlan Supergiant. Part of the oil- and gas-producing zone of the Volga-Urals region is reproduced with permission from the 1:2,000,000 map infolded in Trofimov (2014).
Urals and the Middle-Urals Ring Structure (MURS) – coloured gemstones and more
According to Burba (2003a), “The overwhelming majority of the mineral fields of the Ural Mountains are concentrated within the [>400km diameter] MURS, or to put it more precisely, within the eastern half of its rim. There are ore fields of iron, copper, chromite, nickel, tungsten, gold, and some other metallic mineral resources here, as well as of the well-known Ural gems [italics added]. One could say, that the mining industry of the Ural is in intimate connection with MURS. So, MURS makes the industrial power of the Urals”. Burba’s various sketch maps and descriptions (Burba 1991, 2003a, 2003b) do not allow the MURS to be unambiguously plotted on a detailed map. But the Oil & Gas Map infolded in Trofimov (2014) shows the 210km diameter Romashkino Dome as tangent to the MURS, with the 90km-long Arlan Supergiant oilfield astride the common tangent. This then permits a satisfactory mapping of the MURS (Figure 4; also shown on Figure 3, across eastern Europe diagonally opposite the Bohemian Massif). The gems were formed in conjunction with the collision of Europe with Asia, but the manner and depth at which the Romashkino structure came into contact with the MURS, and their relationships to the Middle Urals Mountains themselves, remains to be resolved.
Hindu Kush and Pamirs – coloured gemstones
The Pamir Mountains and the Hindu Kush (Figure 5) possess circular cores with diameters of ~640km and ~290km respectively. Both host multiple primary deposits of transparent coloured gemstones, formed in conjunction with the continent-to-continent collision of peninsular India with continental Asia (Saul, 2017). Near Jegdalek, Afghanistan (indicated by the red star to the east of the southern intersection of the two circles), a dozen or more ruby occurrences are aligned tangentially to the Pamir circle. Eastern
Kenya and Tanzania – tanzanite, rubies, and other coloured gemstones
In the eastern Kenya-Tanzania border region, deposits of hard transparent gemstones are associated with a circular feature 255km in diameter (Figure 6). These are discussed in Saul (2022) in which the parallel faults are interpreted as oriented perpendicular to a gem-forming continent-to-continent collision that occurred approximately 650 to 620 Ma. In the northeast quadrant, the circumference of the circle is followed by the course of the Mwatate River. A much smaller circular scar, approximately 5km in diameter, is situated on the rim zone of the 255km circle and is associated with the Umba River ultra-basic intrusion, source of pastel sapphires, umbalite garnets, and other gem varieties, many of which are also recovered from the gravels of the Umba River.

Figure 5. Gemstone locality map of the Pamir Mountains and the Hindu Kush, both of which have circular cores (from Saul, 2017). GPS velocity field motion vectors relative to stable Eurasia from Gan et al. (2007). Base map by Geo-Innovations Ltd.

Figure 6. Map of circular scar, 255km in diameter, with which many of the gemstone occurrences of the eastern Kenya-Tanzania border area are associated. There is a much smaller circular scar (shown in grey on the main map), approximately 5km in diameter, where its rim zone is crossed by the Umba River. The Umba River ultrabasic intrusion appears to be located close to the common crossings of both circles with the river. Sections of the Mwatate and Umba rivers are emphasized in blue. The solid red lines indicate faults shown on diverse published maps. Dashed red lines are proposed faults.

Figure 7. False colour satellite image of the Mahenge gem-rich karst area in south central Tanzania. Numerous circular scars are visible, a few of which have been outlined.
Mahenge, Tanzania – spinel, garnets, and other coloured gemstones
The Mahenge area (Figure 7), source of fine spinels and other gem varieties, is situated more than 500km south-southwest of the Umba River. Gems are found in hardrock and in secondary deposits that include sinkholes and other karst features. Multiple circular scars are present in the area, made visible by the same dissolution and erosion that formed the karsts. Valleys south and south-southeast of Mahenge town are particularly rich in gems.
A dashed red line has been added to the right-hand image in Figure 7. Despite appearances, this line was not drawn to represent the axis of the fold structure that dominates the figure. Instead, it was copied from the parallel faults mapped in Figure 6 and transferred to the Mahenge area more than 500km to the north-northeast, an indication of the great extent of the gem-forming collision zone.
Concluding Remarks
Numerous occurrences of transparent gemstones, including both coloured stone deposits and diamond-bearing kimberlites, are associated with unambiguously circular scars. These are surface expressions of deep structures, elements of what Jerome and Cook (1967) called the Earth’s “deeply penetrating fundamental ‘plumbing’ system”. The diameters of these circular structures range through four orders of magnitude, from a few kilometres to a few thousand kilometres, leaving the bombardment that also scarred the Moon as the only candidate mechanism available to account for their existence (Saul, 1978).
Current versions of plate-tectonic theory cannot accommodate these observations and will have to be modified. Will the tail of gemmology wag the dog of geology?
References
Burba, G.A., 1991. Middle-Urals Ring Structure, USSR: Definition, description, possible planetary analogues. Lunar and Planetary Science Conference, 22, Abstracts, p. 153.
Burba, G.A., 2003a. Effects of the supposed Giant Impact Crater on the geological evolution of the Ural Mountain Range. International Conference on Large Meteorite Impacts, 3, Abstract 4117.
Burba, G.A., 2003b. The geological evolution of the Ural Mountains: A supposed exposure to a giant Impact. Vernadsky/Brown Microsymposium, 38, MS011, http://www.planetary.brown.edu/planetary/international/Micro_38_Abs/ms011.pdf (retrieved 7 Dec. 2016).
Gan, W., Zhang, P., Shen, Z.-K., Niu, Z., Wang, M., Wan, Y., Zhou, D., and Cheng, J., 2007. Present-day crustal motion within the Tibetan Plateau inferred from GPS measurements, Journal of Geophysical Research, 112, B08416.
Hartmann, W.K., 2010. Craters, first significant figures, and the pivotal year of 4,000,000,000 B.C.E. Meteoritics & Planetary Science, 45 Supplement (Abstract 5052) p. A77.
Jerome, S.E., and Cook, D.R., 1967. Relation of some metal mining districts in the western United States to regional tectonic environments and igneous activity. Bulletin of the Nevada Bureau of Mines, 67, 35 pp.
Ryder, G., Koeberl, C., and Mojzsis, S.J., 2000. Heavy bombardment of the Earth at ~3.85 Ga: the search for petrographic and geochemical evidence. In Canup, R.M., and Righter, K., eds., Origin of the Earth and Moon, Tucson: University of Arizona Press, pp. 475-492.
Saul, J.M., 1978. Circular structures of large scale and great age on the Earth’s surface. Nature, 271, no. 5643, pp. 345-349.
Saul, J.M., 2014. A Geologist Speculates,
Les 3 Colonnes, Paris, 159 pp., ISBN 978-2-37081-004-5
Saul, J.M., 2017. Transparent gemstones and the most recent supercontinent cycle. International Geology Review, 60(7), pp. 889-910.
Saul, J.M., 2022. Gemstone Deposits of Eastern Kenya and Tanzania Controlled by Ancient Meteorite Impacts and Continental Collision – an Exploration Model, The Australian Gemmologist, 28(1), pp. 15-24.
Trofimov, V.A., 2014. Deep CMP Seismic Survey of Oil and Gas Bearing Areas (in Russian), GEOS, Moscow, 202 pp. with 1:2,000,000 infolded map