Emeralds from Sandawana
Emeralds from Sandawana
By E. J. Gübelin, Ph.D., C.G., F.G.A.
The following article appeared in The Journal of Gemmology, Vol. VI, No. 8, October 1958, and is reprinted by permission.
It is most pleasant and gratifying that even in this atomic age of sophistication and technique, which seems to foresee everything by means of reason and computation, the realm of gemstones has not lost its nimbus of romance and adventure and that now and then new gems or new gem localities are found.
In October, 1956, the prospectors, Laurence Contat and Cornelius Oosthuizen, who had been attracted by the advantageous offers made to prospectors by the Rhodesian Government, happened to investigate the rocks near the valley of Sandawana in the Belingwe district of Southern Rhodesia together with their "prospecting boys"—native helpers—trained to look for unusual rock yields. Having a good knowledge of geology, Contat and Oosthuizen decided to concentrate on a small area showing the most favourable conditions for the occurrence of large pegmatites. They were not disappointed. Within ten days after their arrival they discovered the first emeralds and they ordered their boys to fan out in a more intensified search, but it was seven months before they struck the second deposit—a rich one this time.
The first samples were taken to Mr. A. E. Phaup of the Geological Survey Office at Gwelo, who confirmed the gem's nature. The Department of Mines was then informed, but the Southern Rhodesian Government was not very much interested in this find until they heard that Contat and Oosthuizen had sent emeralds worth about US$15,000 to New York. An ordinance was then passed in February, 1958, by which all precious stones in Rhodesia were subject to the same control regulations as those in South Africa. At first the rumours mentioned a diamond find, but when the law was gazetted on 25th April, 1958, it was known everywhere that emeralds had been found.
The writer was then asked to investigate these newly found emeralds in order to find out their local peculiarities and other typical characteristics that would distinguish them from emeralds from the already known sources which have been thoroughly examined and are known to well-informed gemmologists. An initial lot of 92 stones was submitted to him and it appeared appropriate to carry out an extensive study so that comparison could be made between these new emeralds and those of other provenance with the hope of ascertaining some locally typical features which might differentiate them from others.
The emerald occurrence of Sandawana is in very isolated country on the south side of the Mweza Range of mountains and so far only little geological and petrological investigation has been carried out. The position is roughly longitude 29° 56' east and latitude 20° 55' south. The Mweza Range is composed of very old pre-Cambrian rocks extending for some 45 miles (72 km.) east-north-east in a huge area of granitic rocks that are also of pre-Cambrian age. The range is two to three miles wide (3 to 5 km.) and rises about 500-800 ft. (150 to 200 m.) above the surrounding country.
The metamorphic rocks forming the range belong to the Bulawayan System of Southern Rhodesia and are probably 2,700 to 3,100 million years old. They have been tightly folded into a complex structure with a general direction east-north-east and very steep, almost perpendicular. On the flanks of the range the rocks are basaltic lavas and dolerites that have been metamorphosed into greenstones and epidiorites composed of amphiboles (chiefly hornblende), sodic feldspar and clinozoisite. The core of the range is a variety of banded sedimentary rocks that have been metamorphosed and now contain micas, almandine garnet and various amphiboles, including grunerite. They consist of banded ironstones, phyllites, sericite-quartz schists and, quartzites. Very long, narrow sills of peridotites were intruded into these older rocks and have been altered into serpentine and related rocks.
The rocks forming the range were folded, metamorphosed and intruded by granitic rocks at the end of the Bulawayan times and again at the end of Shamvian times about 2,650 million years ago. A batholitic mass of granitic rocks extends for over 20 miles (32 km.) in all directions from the range and small stocks were intruded into the schists of the range and the gneissic granite contact around it. The emeralds have been found associated with the pegmatite dykes in the tremolite schists. The majority of the dykes probably belong to the end of the Shamvian System and contain beryl, lepidolite, petalite, spodumene and tantalum-niobium minerals. The intrusion of the granitic magma into the tremolite schists caused contact metamorphosis of the two rocks through which new minerals were formed, the most valuable of which is the emerald. It is most instructive to study the formation of emerald as shown by this particular deposit, which seems to have much in common with the emerald-bearing rocks at Tokowaya (Ural), Transvaal, Ajmer in India and the Habachtal in Austria. There, same as here, the circumstances of formation and the chemical composition of the various rocks explain the formation of the emerald. The beryllium is an element that emerges from the granite. The tremolite schists do not carry any beryllium. The granite has also brought free silica, while chromium, which acts as a pigment, under favourable conditions accompanies basic rocks of the gabbro group, which are poor in silica ; it is present in minute quantities in the altered serpentine rocks and was carried up from great depths by the peridotite. In the course of the numerous successive intrusions, alterations .and re-formations, the chromium got into the process of contact-metamorphosis, thus becoming responsible for the superb colour of the Sandawana emeralds.
Up to the day of writing these notes, no clearly and well developed crystals could be obtained from the new mine, but a report from Sandawana mentioned that well developed, euhedral crystals are extremely rare and that most of the crystals found are so thickly encrusted with limonite that no clear crystal faces nor their combinations could be observed, let alone measured. Therefore, the present examination of Sandawana emeralds could not include a study of the crystal habit and its possible local peculiarities. Of the 92 cut specimens received for carrying out the study, all displayed a superb verdant emerald green with a brightening yellow glint that renders the stones very vivid. To the naked eye the gems appear amazingly clean and only in a few specimens denser concentrations of inclusions altered the colour into a more bluish green hue.
Unfortunately the majority of the rough material is so badly broken or occurring as small crystals only, that most of the Sandawana emeralds reaching the market of cut gems will be below 1/4 carat in weight, although larger gems may appear as the exploitation of the vast deposit progresses. The largest Sandawana emerald obtained to date weighs 1·56 carat as a cut gem. It is, however, welcome news to the jeweller that even small calibre sizes, weighing only a few points, retain the unrivalled beauty of colour, which seems to be the outstanding virtue of the Sandawana emeralds.
A small amount of rough material made it possible to analyse the chemical composition, which was determined jointly by a chemical as well as a spectrographic analysis. On account of the tiny quantity at the writer's disposal, the three main components had to be· determined with a piece weighing as little as 50 mg., which somewhat impaired the accuracy to be expected. (It may be mentioned that even with normal quantities of 0·5–1·0 gm. the analysis of beryl is no easy task.) The chemical analysis supplemented by the spectrographic examination gave the following result:—
The figures in parenthesis [sic] represent the theoretical values based on the formula:—
SiO2 was determined by evaporating twice with HCl
Al2O3 was precipated as oxychinolate from an acetic solution
BeO was separated by means of NH3 with pH 8–9
Cr, Fe, Mg, Na and Li were ascertained by means of a Jarrell-Ash Ebert plane grating spectrograph with direct current carbon arc and anodic irritation. The spectral lines used were: Cr 5204, Fe 4325, Mg 5167, Na 5688, Li 6103. The accuracy is 10–20 per cent of the result obtained. It is interesting that a relatively remarkable quantity of lithium is present, the percentage of which amounts to ·15 per cent.
Of the 92 cut gems submitted, ten specimens of outstanding quality were selected, so that the greatest possible constancy of data was guaranteed. Table I shows the individual data obtained and comparison with the individual figures shows how minute the variations are.
The refractive indices were determined with an Abbé-Pulfrich total reflectometer which enables four decimals to be read. The readings are collected in Table I and it may be noticed that they vary individually between the extreme values of 1·5877–1·5949 for ω and from 1·5806 to 1·5884 for ε: with a slightly varying birefringence from ·0069 to ·0071.
With the intention of receiving even more reliable constants and simultaneously measuring the dispersion, the optical properties of a small, clear crystal prism were measured by means of the method of minimum deviation. A one-circle goniometer was set up in combination with a monochromator. The crystal prism was extraordinarily euhedral and displayed very flat and smooth prism faces, so that values of high accuracy could be expected. First the medium birefringence was determined from a series of measurements which showed little variation only and the arithmetical average was found to be ·0071. Then the mean value of the various individual measurements of w and e: was determined for each of the five wave-lengths that had been used and these values were then balanced with the average birefringent constant. This procedure resulted in the optical data compiled in Table II (the figures have been rounded off to the third decimal).
None of these average figures differs by more than ±·0002 from those actually measured. Hence the variation is very small. Perhaps one ought to have reckoned not only with a dispersion of the refractive indices (n), but also of the birefringence (Δ). However, since no systematic alteration of the Δ-values according to the wave-lengths could be observed, an effect of this kind (if any at all) may be too small to be noticed. Consequently, it may be assumed that the adjustment of the birefringence to ·0071 over the whole spectrum will hardly cause any mistake. Thus the most reliable figures for the optical data of emeralds from Sandawana can be given as:—
nω = 1·593 ; nε = 1·586 ; Δ = ·007
From Table II the reader may also derive that the relative dispersion B-F of this new emerald amounts to ·009. Comparison with Table I shows a rather good concurrence of these R.I. values with those of specimens Nos. 5, 6, 8 and 10. For further comparison emerald No. 5 was also examined by the method of minimum deviation and a similarly good agreement was obtained as the following figures were measured for the D-line:—
nω = 1·592 ; nε = 1·586 ; Δ = ·006
with a relative dispersion B-F of ·0085. At this place it may be interesting to compare the optical values of the Sandawana emeralds with those of other important sources as registered by Ph. Vogel, who probably carried out the most thorough study on the optical properties of emeralds from the main sources known then. In Table III the refractive indices of the Rhodesian Stones are the highest for all wavelengths. The emeralds from the Habachtal, the Ural, Transvaal, Colombia and Brazil follow with degrading values. The same diminution occurs with the birefringence. While the progress of the dispersion remains identical through all
the wave-lengths for the emeralds from different sources, the numerical amount of the relative dispersion (B-F) varies in being similar for emeralds from Rhodesia, Colombia, Brazil and the Habachtal on one side and for those from the Ural and Transvaal on the other.
With regard to the other optical properties, the Sandawana emeralds do not seem to show any anomaly or local peculiarity but appear to behave very similarly to their relatives from other deposits. Dichroism is distinct-displaying yellow-green for ω and bluish-green for ε. When viewed through the Chelsea colour filter the gems appear very weak red.
In the absorption spectrum the normal absorption lines show clearly at 6830, 6800, 6620, 6460, and 6370Å, as well as the characteristic band extending from 6300–5800Å with its absorption maximum near 6125Å, while the minimum is in the region of 5050Å. The absorption lines at 6370 and 6460Å usually just form the stronger border lines of the band extending between them. In one stone only the line at 6460Å was observed.
Although Sandawana emeralds do not react in the least when excited with u.v. light of short or long waves, it may easily be noticed that the stones are remarkably less transparent in short u.v. rays, and indeed examination with the u.v. absorption spectroscope revealed that they transmitted u.v.-light only down to 3200Å where complete absorption set in with a sharp cut off As far as this phenomenon is concerned Sandawana emeralds behave like those from India. In the Stokes' fluoroscope (two-filter method) all specimens tested displayed a distinct fluorescence of glowing pale red colour.
The figures of the specific gravity were ascertained by means of the hydrostatic method, immersing the stones in ethylene dibromide and weighing very carefully with a semi-automatic Mettler balance Although all the stones examined were rather small, the remarkable freedom from any influential amount of inclusions ensured a rather good consistency of constants between the extreme values of 2·744 and 2·768 with a mean figure of 2·756. Thus the specific gravity of Sandawana emeralds ranks among the highest density figures known for emeralds and stands in conformity with the high refractive indices and great birefringence.
All the properties described above, though ranging among the highest ones for emeralds-are not sufficiently individual to serve as marks of identification of Sandawana emeralds or to distinguish them clearly from those of other localities. The constants of emeralds overlap in border cases, rendering it impossible to determine the origin of a given specimen, were it not for the unique internal birth-marks which in most cases point infallibly to the place of formation.
It was mentioned before that the Sandawana emeralds resulted from contact-metamorphosis between granitic magma and tremolite schists, and, as a matter of fact, acicular tremolite inclusions yield the most characteristic feature of their internal paragenesis. The gems of inferior quality teem with dense masses of short and long slender needles (Fig. 1), while in those of good quality the tremolites occur either as short pins lying criss-cross, or as very fine long fibres assembled as dense bundles or masses without any definite orientation at all (Fig. 2). Sometimes the short pins are brownish but usually all the tremolite needles are bluish-green, throwing upon the host emeralds a slight bluish cast when present in great quantities. Quite often these fibres are curved and sometimes they run through several disc-like cleavage fissures, leaving the impression of tiny flakes being threaded on silk.
The phenomenological picture of these tremolite inclusions resembles somewhat that shown by the emeralds from the actinolite schists in the Habachtal, and to those seeing it for the first time it may be most confusing. In Sandawana emeralds, however, the tremolite fibres are much finer, the occurrence of dense masses is rare and there are always short pins present (Fig. 3). Indeed, in the Rhodesian stones the tremolite fibres rather form bundles and are usually so fine that they may be compared with the well-known byssolite fibres in demantoid.
Already considerably more vivid and beautiful by nature than the emeralds from the Habachtal, the Sandawana emerald will always excel by its superior splendour and greater transparency since the tremolite needles as well as other inclusions are much rarer. The deposit in the Habachtal is almost exhausted and has never thrown many stones of good quality on the gem market, so that in future emeralds containing tremolite needles may with the greatest safety be considered as Sandawana emeralds because no other important ·emerald source is known to have its precious products teeming with tremolite fibres.
The tremolite needles are usually associated with other endogenetic minerals of minor importance. The most frequent and typical companion is a brown, limonitized garnet which usually sits in cracks near the surface of the host gem (Fig. 4). The cracks are in most cases filled with limonite, which is quite common and also coats the surface of the emeralds. In association with garnets it often forms brownish, patchy halos, which also seem to be a local characteristic, yet considerably rarer than the tremolite needles. Apart from these, other rare inclusions consist of hematite tablets, decomposed plagioclase feldspar and dots of magnetite. Anthophyllite is present in the wall rock neighbouring the emerald-bearing matrix, so that it is possible for anthophyllite needles to occur as inclusions, but none have so far been seen in the gems tested.
Yet the interior of the Sandawana emeralds is not only typified by solid inclusions-two most individual kinds of inhomogeneities add to its character. One gives the impression of being splashes of a dust-like appearance, oriented parallel to the C-axis. Normally they seem to be green but sometimes they may be limonitized and then assume a brownish tint (Fig. 5). Occasionally it is impossible to distinguish these splashes from the brownish halos surrounding the garnet inclusions. Just as strange as these are sheets consisting of irregular, interrupted lines and strokes running more or less parallel (Fig. 6). They represent systems of very fine cleavage fissures lying parallel to the basal plane. Although resulting from exactly the same cause of origin—a deficiency of cohesion between the sheets of (SiO3)6 rings—as those well known and characteristic planes of disc-like fissures which are so abundant in emeralds from the Urals, the pattern of these cleavage fissures is distinctly different and should not lead to confusion. In Russian emeralds they appear like silvery fish-scales lying in flat or curved layers, while in the Rhodesian stones they resemble irregular strokes with a brush.
One specimen displayed beautiful, lace-like patterns of liquid films which have remained "undigested" in healing fissures (Fig. 7). One single occurrence of such formations does not justify a generalization of the observation and, indeed, it may not be a typically local feature, but it should be noted and may serve as a valuable reference for future investigations of Sandawana emeralds.
It will be interesting for the reader to get acquainted with Sandawana emeralds ; he will not find it difficult to recognize their locally typical inclusions and to distinguish them from the emeralds of other occurrences.
The author wishes to express his gratitude to Mr. A. E. Phaup of the Geological Survey Office at Gwelo for his explanations of the geological conditions, to Prof. R. L. Parker for the determination of the R.I.'s by the minimum deviation method, and to Dr. M. Weibel for the chemical analysis.
R. Webster. The Emerald, Journ. Gemmology, October, 1955, p. 185.
Dan E. Mayers. The Sandawana Emerald Discovery, Gemmologist, March, 1958, p. 39.
E. J. Gübelin. Emerald from Habachtal, Journ. Gemmology, Vol. V, No. 7, 1956.
E. J. Gübelin. Some additional data on Indian Emeralds, Gems and Gemology, Spring, 1951, p. 13.
B. W. Anderson. A New Test for Synthetic Emeralds, Gemmologist, July, 1953, p. 115.
Philipp Vogel. Optische Untersuchungen am Smaragd und einigen anderen durch Chrom gefärbten Mineralien, Neues Jahrbrch fur Mineralogie Beil, Bd. 68, Abt. A 1934, p. 401–438.