If the stone is massive, eg. multi-micro cystalline, it’s not a single crystal. What you have is not sapphire of gem quality but is an unusual and somewhat rare collector’s specimen of massive corundum. The provenance as widely stated is afganistan/pakistan… a too large area to be more specific. The general geology is metamorphic rock of medium to high grade containing a variety of pre metamorphic rocks ranging from ultramafic to highly felsic…Your rock as JC Bell states is from a vein filling in a medium to high grade metamorphic host rock high in alumina, low in silica. It was an orogenic fluid transport mechanism that crystallized an unusual pegamatitic rock. Retrograde metamorphism as the rock slowly cooled could have precipitated calcium mica. Calcic mica itself is quite unusual. It can form from the dissolution of aluminum containing minerals, including corundum itself but more commonly from andalusite. It seems like you’ve already spent a lot of time and money on trying to identify that rock. It’s still a collector’s specimen that could be quite valuable in it’s uniqueness but not a gemstone. If you want to pursue it’s chemistry and structure and spend more money and time, you might consider taking it to a academic/university genochemistry department for further inspection and analysis. This would be a purely academic proposal. However, depending on where you are living, it could be much easier. If you are in France, as you found it a Paris flea market, the there are many resources.avaiable. Contact European Geochemical Society website to find thier affliated organisations some located within Paris itself. That’s only if you’re not completely satisfied with your own paid for analyses already,
golden sheen sapphires according the GIA article contain oxidized iron minerals. this is most consistent with “basaltic” sapphires. I do think Kadda has got a collector’s item. A pegmatitic corundum is itself unusual. The accessory mica minerals Ca/Na containing “brittle micas” result from the substitution of Al in place of Si in the sheet structure of the mica. This would point to desilification of a felsic pegmatite and or fluid intruded into an ultramafic rock or a serpentinite…
from all of the discussion, I know that you have found something very interesting and also fairly uncommon. By the accessory minerals- the Na/Ca mica it’s most likely a desilicated pegmatitic fluid that precipitated the corundum. The mica is also most likely margarite. This kind of pegmatite is known as Marundite. Best wishes and let us know if you decide to pursue the petrogenesis and geochemistry of your specimen any further.
Ore Geology Reviews
Volume 96, May 2018, Pages 72-97
Formation of corundum and associated mineral zones in the hybrid ultramafic-pegmatite association of the Neoproterozoic Hafafit core complex, South-Eastern Desert, Egypt
Author links open overlay panelAhmed Hassan Ahmed a b, Moustafa Esmail Gharib b
Un trésor caché dans une roche que l’on croyait connaître**
J’ai acheté cette roche par simple curiosité. Ce qui m’a attiré, c’était son poids inhabituel. J’ai sorti mes clés pour voir si elle était tendre, mais ce n’était pas le cas. Alors je l’ai achetée.
Quelques jours plus tard, je suis allé voir des vendeurs en minéralogie. Leur verdict : « C’est de la sodalite ». Ce fut une déception… Je me sentais floué, vu la différence entre le prix payé et celui d’une simple sodalite.
Pour en avoir le cœur net, j’ai décidé de couper un morceau. Le but : mesurer la densité. Résultat : 3,8–3,9. Ce n’était pas tout à fait celle de la sodalite. J’ai donc déposé un échantillon au Laboratoire Français de Gemmologie (LFG). Verdict : saphir basaltique.
Avec ce résultat, je suis retourné voir un directeur de musée de minéralogie. Sa réponse : « C’est un saphir basaltique courant, aux USA ça vaut 100 $ le kilo ».
Peu après, inspiré par le Millennium Sapphire et l’arrivée des Jeux Olympiques à Paris, j’ai voulu réaliser une sculpture commémorative. Pour cela, il me fallait un scan 3D de la pierre. Les photographes étant trop chers, j’ai contacté une société à Lyon (3D Casting) pour faire une tomographie à un prix plus abordable.
C’est après cette tomographie que j’ai remarqué, sur la coupe, des lignes réfléchissantes. Intrigué, j’ai publié les photos sur un groupe de minéralogie. Certains pensaient à des rayures de sciage, d’autres à du rutile pouvant donner un astérisme (étoile). À ce moment-là, je ne connaissais même pas le saphir étoilé.
Comme je ne trouvais pas de réponse claire, j’ai poursuivi avec des analyses microscopiques. Elles ont confirmé que la roche était bien un saphir, mais que ces lignes étaient dues à des inclusions de mica, alignées dans la masse.
Dans l’espoir de tailler un saphir étoilé, j’ai cherché un lapidaire capable de travailler un spécimen aussi gros. Finalement, j’ai pris rendez-vous à Idar-Oberstein avec le fils de M. Helmut Wolf. C’est là que nous avons découvert que la roche se composait d’un bloc de saphir de près de 4 kg, soudé à une gangue polyminérale (feldspath, mica et saphir). Double déception : la pierre n’était pas entièrement en saphir et l’effet étoilé n’apparaissait pas.
De retour à Paris, j’ai rencontré une responsable de laboratoire. Pour elle, l’astérisme ne pouvait venir que du rutile ou de l’hématite, jamais du mica. Elle m’a même conseillé d’en faire un cale-porte… Ce fut un coup dur.
J’ai alors commencé à chercher des informations sur les saphirs à mica, mais j’ai vite constaté qu’il n’existait presque rien à ce sujet. C’est là que, sur le forum, StevenH26783 m’a donné une piste, et que Troyes a également échangé avec moi, me redonnant un certain espoir.
Aujourd’hui, je poursuis mes recherches avec l’aide de l’IA, pour comprendre comment et dans quel environnement un saphir et du mica peuvent cristalliser simultanément.
StevenH26783 et Troyes
Je suis ravi et heureux de partager mon expérience avec vous deux, car vos commentaires m’ont motivé à poursuivre l’enquête sur ce spécimen.
Voici donc ma réponse point par point.
- Nature polycristalline de la pierre
Le spécimen en question est bien polycristallin, c’est-à-dire constitué de plusieurs cristaux de corindon soudés entre eux, formant une masse compacte. Les zones bleues profondes correspondent à du saphir (corindon bleu) et les zones blanches laiteuses correspondent également à du corindon, mais sans coloration notable. L’ensemble montre une continuité cristalline interne, ce qui explique l’aspect massif. Cette structure polycristalline se distingue clairement d’un monocristal (forme unique et régulière).
2. Comparaison avec les saphirs d’Afghanistan
Nous avons effectué une comparaison visuelle et analytique (à partir de nos données EDS/SEM) entre ce spécimen et certains corindons massifs provenant de gisements afghans. Bien que certains points (texture, couleur, morphologie des grains) puissent présenter des similitudes, le spécimen étudié présente des différences notables : homogénéité chromatique au cœur, alignement des inclusions micacées, absence de feldspath au sein de la masse principale, et veines blanches composées elles aussi de corindon.
3. Intervention et remarques de M. Gaston Giuliani
M. Gaston Giuliani, directeur de recherche émérite à l’IRD et spécialiste mondial des gisements de pierres précieuses (corindon, émeraude), aujourd’hui à la retraite, a été sollicité en amont de nos travaux. Lors du premier contact, il a précisé qu’il ne pouvait pas répondre en détail du fait de sa retraite. Après un mois, un second échange a permis d’obtenir des hypothèses concernant l’environnement géologique probable du spécimen, notamment un contexte pegmatitique ou métamorphique particulier. Ces éléments confirment l’intérêt scientifique potentiel de cette pièce.
4. Caractères exceptionnels du spécimen
Plusieurs éléments confèrent à ce saphir massif un caractère unique :
- Morphologie xénomorphe : absence de forme cristalline régulière, ce qui témoigne d’une croissance contrainte par l’environnement.
- Homogénéité chromatique : un bleu nuit intense et constant dans la masse centrale, rare sur un corindon polycristallin de cette taille.
- Contact avec la gangue : une face conserve le point d’attache à une masse feldspatho-micacée, preuve d’une co-croissance minérale.
- Inclusions de mica alignées : créant un effet de « mica-sheen » discret mais visible, renforçant l’attrait visuel.
- Projet de taille expérimentale : possibilité d’extraire des pierres polies présentant un reflet scintillant discret, inédit pour un spécimen de cette masse (plus de 2 kg après taille).
Hello troy and StevenH26783
I am currently comparing my specimen with several well-known deposits, including Kenya, Egypt, Montana (USA), Afghanistan, and Iran.
There are indeed similarities in certain aspects — for example, the polycrystalline texture, the presence of oriented inclusions, and some optical features — but it is not 100% identical to any of them.
One important feature of my specimen is that the mica-sheen it displays is not blue:
- The “mica-sheen” effect is created by mica lamellae (muscovite/phlogopite) that reflect light with silver to golden flashes, depending on the angle and lighting conditions.
- The almost parallel alignment of these lamellae within the sapphire produces a unique visual effect, entirely natural.
Vous m’avez ecrit en francais. J’ai recu une courielle de votre aventure avec cette pierre tres interessante. Je pouvais comprendre presque tous que vous m’avez ecrit. Il y a des mots de vocabulaire dont je ne suis pas tout familaires. Mais en tous cas, j’ai fait une traduction avec Deep L translate. Je ne suis pas assez fluide avec la langue francaise a repondre en bon francais ecrit. Donc je vous ecris maintenant en Anglais.
Your mineral specimen is really intriquing. It’s geochemistry should be investigated further. Basaltic sapphire is not geochemically consistent with calcic mica. A desilicated pegmatite orginiating from a felsic intrusion into mafic gneiss or mafic/ultramafic rock seems to me to be the most likely origin. One of the articles below measures Ga/Mg ratios. Basaltic corundum has much higher Ga/Mg ratios than metamorphic. This article is helpful…Oxygen isotope ratios may also another discriminant…The next step would be LA-ICP-MS which would give a lot more information… below are some references on basaltic sapphires… there is more, but I couldn’t copy and paste all of them…Your idea of having it carved is a great idea. Bonne journee et merci bien de me repondre.
Russian Geology and Geophysics
Volume 55, Issue 9, September 2014, Pages 1087-1102
Mineral inclusions in sapphire from the basalt-related deposit in Bo Phloi, Kanchanaburi, western Thailand: indication of their genesis
Author links open overlay panelP. Khamloet a, V. Pisutha-Arnond a b, C. Sutthirat a b
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Abstract
The Bo Phloi gem field in Kanchanaburi Province, Western Thailand, is closely associated with Cenozoic basalts. Blue and yellow sapphire, black spinel, and minor zircon have been mined for over three decades. The mineral inclusions observed in sapphire samples are alkali feldspar, nepheline, hercynitic spinel, zircon, manganiferous ilmenite, silica-rich enstatite, almandine–pyrope garnet, monazite, calcite, sapphirine, biotite–phlogopite mica, and staurolite. Based on their geochemical affinity, these mineral inclusions can be categorized into two main groups: felsic alkaline and contact-metamorphic, which appear to have originated from different processes. These inclusions provide new evidence for proposing a bimodal genetic model. Felsic alkaline origin is evidenced by the occurrence of a felsic alkaline inclusion suite and the REE geochemistry of sapphire-associated zircon, which indicates that most of the sapphires crystallized from a high-alkali felsic melt (probably, in the lower crust). Contact-metamorphic origin is evidenced by the presence of a contact-metamorphic inclusion suite, suggesting that some of these sapphires might also have originated from metasomatized crustal rocks and a contaminated melt along the contact zone of a basaltic intrusion (probably, in the upper mantle or lower crust).
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Cited by (25)
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Origin of sapphires from a lamprophyre dike at Yogo Gulch, Montana, USA: Clues from their melt inclusions
2016, Lithos
Show abstract
Gem corundum (sapphire) has been mined from an ultramafic lamprophyre dike at Yogo Gulch in central Montana for over 100 years. The sapphires bear signs of corrosion showing that they were not in equilibrium with the lamprophyre that transported them; however, their genesis is poorly understood. We report here the observation of minute glassy melt inclusions in Yogo sapphires. These inclusions are Na- and Ca-rich, Fe-, Mg-, and K-poor silicate glasses with compositions unlike that of the host lamprophyre. Larger, recrystallized melt inclusions contain analcime and calcite drawing a striking resemblance to leucocratic ocelli in the lamprophyre. We suggest here that sapphires formed through partial melting of Al-rich rocks, likely as the lamprophyre pooled at the base of the continental crust. This idea is corroborated by MELTS calculations on a kyanite-eclogite protolith which was presumably derived from a troctolite precursor. These calculations suggest that corundum can form through peritectic melting of kyanite. Linking the melt inclusions petrologically to the lamprophyre represents a significant advancement in our understanding of sapphire genesis and sheds light on how mantle-derived magmas may interact with the continental crust on their ascent to the surface.
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Zircon xenocrysts from cenozoic alkaline basalts of the ratanakiri volcanic province (Cambodia), southeast asia—trace element geochemistry, O-Hf isotopic composition, U-PB and (U-TH)/he geochronology—revelations into the underlying lithospheric mantle
2018, Minerals
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A common origin for Thai/Cambodian rubies and blue and violet sapphires from Yogo Gulch, Montana, U.S.A.?
2018, American Mineralogist
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Petrogenesis of alkaline basalt-hosted sapphire megacrysts. Petrological and geochemical investigations of in situ sapphire occurrences from the Siebengebirge Volcanic Field, Germany
2017, Contributions to Mineralogy and Petrology
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Major zircon megacryst suites of the Indo-Pacific lithospheric margin (ZIP) and their petrogenetic and regional implications
2016, Mineralogy and Petrology
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Sapphire within zircon-rich gem deposits, Bo Loei, Ratanakiri Province, Cambodia: trace elements, inclusions, U–Pb dating and genesis
2015, Australian Journal of Earth Sciences
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Geochemistry of tourmaline of elbaite-dravite series from sapphire bearing pegmatites, proterozoic higher Himalayan Crystalline Complex Jammu and Kashmir, India: Implication for evolution of pegmatite melt
Author links open overlay panelPankaj K. Srivastava, Pawan Singh
Cet article sur les tourmalines contenues dans les pegmatites métamorphiques s’applique également au saphir. La collision continentale entre l’Inde et l’Asie a entraîné la fusion partielle de la croûte sédimentaire pélitique. Il existe une ceinture de leucogranites contenant des pegmatites riches en gemmes. L’intrusion de granite dans les roches ultramafiques avec métasomatisme a augmenté la teneur en aluminium du fluide résiduel à un niveau suffisamment élevé pour permettre la cristallisation du corindon. Contrairement aux leucogranites eux-mêmes, des éléments mafiques ont été ajoutés aux fluides à cristallisation tardive qui ont été désiliciés au contact des roches ultramafiques. Cela pourrait expliquer la présence de feldspath et de micas calciques dans le saphir. Les saphirs magmatiques ont un rapport Gd/Mg supérieur ou égal à dix. Les saphirs métamorphiques ont un faible rapport Gd/Mg. La spectrométrie de masse serait la prochaine étape pour mesurer les rapports des éléments traces et confirmer que votre saphir provient d’Afghanistan. Bonne chance a votre recherches.
This article on metamorphic pegmatite hosted tourmalines also applies to sapphire. The continental collision belt between India and Asia resulted in partial melting of pelitic sediment crust. There is a belt of leucogranites hosting pegmatites with gem content. Intrusion of granite into ultramafic rocks with metasomatism increased the aluminum content of residual fluid high enough for corundum to crystallize. Unlike the leucogranites themselves, mafic elements were added to the late crystallizing fluids that were desilicated at the contact with ultramafic rocks. This could be an explanation for the feldspar and calcium micas the gauge and within the sapphire. Magmatic sappires have a Gd/Mg ratio of ten or above. Metamorphic sapphires have low Gd/Mg. Mass spectrometry would be the next step to measure trace element ratios and confirm that your sapphire came from Afghanistan. Good luck on your research.
Hello Steven,
First of all, I would like to thank you for your very complete and informative response.
At the beginning of my journey, I only consulted gemologists, but I could not find precise answers. After your advice to also explore the fields of geology, petrochemistry, and mineralogy, my understanding has become much clearer. This encourages me to continue this adventure.
I have already contacted several universities, laboratories, and even two museums. Unfortunately, their replies were often that they were not able to analyze this type of rock… or I simply did not receive any answer. Since it is currently the summer holiday period, I plan to resume my search in September or October to find an institute capable of carrying out the analyses you have recommended.
Regarding the text I published in French on the forum instead of English, it is because I also use a translator to understand and write your messages. I made a mistake and did not want to remove it, but if necessary, I can republish it in English.
As I wrote in my message: a curiosity has made me a researcher. I also want to learn more about this rock, and why not, as you suggested, one day see it displayed in a museum.
I have also discovered the existence of the giant from Mogok, About 1967, a 12.6-kg (63,000 ct) crystal surfaced at Mogok. Today this sapphire colossus is on display at the Myanma Gems Enterprise (MGE) museum in Yangon. on which I am trying to gather information to compare it with my specimen. This also gave me the desire to finally see the results of the tomography, and perhaps consider removing the white layer of feldspar that partially covers the sapphire.
Once again, thank you for your help and the leads you have given me, which have opened new perspectives in this fascinating research.
Best regards,
Kadda,
The sheen effect in your specimen is very unique. In the larger stone, there does seem to be pockets where the stratified mica is less visible and in some areas it doesn’t seem to be visible at all. This could explain the dissenting information between the original lab reports from LFG and CNRS if separate samples were used.
I agree with Steven. It seems the next step would be to have a LA-ICP-MS study done to refine the chemistry. If the two original samples provided to LFG and CNRS were available, those could be used for the LA-ICP-MS, and I think it would provide collaborative evidence.
The tomography study should provide some interesting clues relative to the voids where the mica is not present, and maybe provide a map of the feldspar inclusions.
Another suggestion would be a thin-section petrologic analysis. You have several candidate samples that could be used. Knowing grain-size and structure along with polarimetric stress-loading could give clues to the growth process. It would also be good to hunt for zircons in the matrix which could measure a U-Pb dating.
Interestingly, an article discussing combining thin-section and x-ray tomography was published last year, but somehow found its way into my reading que on LinkedIn today.
I traced the article back to the originator so anyone not on LinkedIn can read it here:
Cheers!
Steven,
These articles are amazing! An absolute gold mine of information! Thanks for post them! Will be heading back to the homestead now that the work project was completed last night. Seems I have a few articles to read on the way home now. 
Kadda definitely has a unique specimen and I agree. It should not become a fancy ashtray!
Cheers!
Apros de moi, ce n’est pas necessaire de le traduire, Je peux comprendre et parler francais assez bien…mais pour les autre sur ce forum, il faut que l’on fasse une traduction. Merci, bien. Si j’ai fait des fautes grammatique ou de l’orthographie, pardonnez moi. (je l’ecis sans traductor IA)
As for myself, it’s not necessary to translate it. I can understand and speak French well enough. But for the others on this forum, it’s necessary to make translation. Thank you very much. If I have made any mistakes in grammar or spelling, please forgive me. (I write this without an AI translator)
I don’t think it’s really one of a kind… but this type of sapphire is very uncommon. Pegmatite hosted meteamorphic sapphires require unusual geological circumstances. From the geochemical perspective further studies will shed light on its petrogenesis. Trace element and isotope ratios can pin it down to a specific area of provenance if there are other samples from Afghanistan to compare with…
Thanks for the read. Micro analysis of rocks and minerals is progressing exponentially, as all of science is… Another example is the modal analysis of a rock in thin section. Identiying and quantitating mineral grains in a rock took hours by visual analysis under polarized light, counting individual grains, estimating the volume by surface area… Now it can be done in a few seconds using AI tools.
Keep the specimen as is, take the slice that you cut from it also. The matrix and gangue material is just as important as the corundum itself… their chemical profile is EXTREMELY important…if you do cut some of it off, keep it in context… where on the specimen it was cut off from…the type of feldspar, its trace element profile, stable and radioactive daughter product ratios, particularly Nd/Sm, Rb/Sr, Uranium to lead/lead isotope ratios if present in measurable amounts will date the rock, as well provide even more information about its petrogenesis…Zircon dating is the gold standard, but if there are no zircons in the rock, it still can be dated…
Hello
In France, no laboratory has the equipment required to analyse a multi-kilogram sapphire block like mine. I therefore proceeded by fragments, carefully documenting each step (photos, videos, certificates, measurements, preserved samples, 3D model from tomography).
After acquisition, some mineralogists/traders thought it was sodalite → I decided to cut a first fragment of 10 g.
From this 10 g, I extracted ~1 ct for the LFG → result: sapphire.
Before polishing, the reflections were already visible. After polishing the remaining piece, they became much sharper, as shown in the previously shared photos.
From that remaining piece, I took a resin-embedded sample** for a single microscopic analysis, aiming to determine the mineral responsible for the reflections. The analyst was not specialised in gemmology or deposits, but rather in biomineralisation; the chosen sample was homogeneous blue, without other minerals, in order to focus exclusively on the reflection phenomenon.
The sapphire is xenomorphic, massive, deep blue, without idiomorphic crystals. It shows a **continuous alignment of mica (striping) from one side to the other, visible in both the blue and white zones. The rock is **polymineral and polycrystalline, with a feldspar–mica–sapphire gangue fused to the main crystal.
Tomography and the part of the gangue still attached to the sapphire show that they share the same origin and formed in the same environment
Some fissures in the sapphire appear to have been filled by the gangue, which remains to be confirmed by analysis.
Work and investments already carried out:
Separation of sapphire and gangue (documented slicing).
Analyses already performed in laboratory and at CNRS.
Preservation of the original samples.
Precise STL 3D model of the original block to reproduce its shape, weight, and colours.
Over one hundred photos from multiple angles before and after cutting.
During initial cleaning with a metal brush, I recovered a white layer mixing earth and mica; metal/sapphire contact produced sparks.
Open question: The next analyses will have to be performed in duplicate (sapphire + gangue). Once the university study is completed, it will be necessary to obtain a certificate from a recognised mineralogical laboratory — probably abroad, since France lacks this capability. Secure transport + insurance will be mandatory (due to its high value). At Gübelin, for example, such an expertise costs at least €3,000.
My question : Do you think it is worth investing several thousand euros or dollars more for these analyses and certifications, given the potential value of the specimen?
If you live in Paris, you could try to find someone at the University of Paris/Sorbonne/Marie and Pierre Curie Institute of Science. La Societe Francaise de Minerologie et Cristallographie . Their address and website is below. The best bet is to find someone there- faculty, grad student or a post doctoral who is working on researching pegmatite hosted corundum…if there’s someone who you can get interested in your rock, they might be able to do the geochemical research on that rock as part of larger geochemical research project on metamorphic pegmatite hosted corundum. The most interesting thing about your rock in the “brittle mica” inclusions. By contacting the SFMC, someone there could lead you to the right person at the Univeristy of Paris Curie Institute… As for myself, this kind of work is only a hobby interest. I’m not a geochemical researcher. I also can’t find any specific references on Calcic mica inclusions in corundum. There are papers that cite brittle mica and corundum co-forming. I can’t find myself any reference on just specifically on brittle mica inclusions in sapphire.
Contactez nous
Le siège de la SFMC est implanté sur le campus Jussieu de l’UPMC, dans les locaux de l’Institut de Minéralogie et de Physique des milieux condensés (IMPMC, ex LMCP).
Adresse postale de la société:
Société Francaise de Minéralogie et de Cristallographie
Case postale 115,
4 place Jussieu,
75252 Paris cedex 05
Here’s a general reference-
Geology of Corundum and Emerald Gem Deposits: A Review
Gaston Giuliani and Lee A. Groat
ABSTRACTPART I: RUBY AND SAPPHIREPART II: EMERALDSUMMARYGlossary
Geology of Corundum and Emerald Gem Deposits: A Review
Gaston Giuliani and Lee A. Groat
I found this on a Google AI search. The refereces cited weren’t specific enough… I don’t know where AI came up with this. Our forum is also being cited
- Inclusions in corundum
- Origin: Brittle mica inclusions can be syngenetic, forming alongside the corundum during its crystallization, or secondary, forming later within cracks or fractures in the corundum.
- Identification: Mica inclusions are often visible as thin, plate-like crystals within the corundum. Some micas can also be identified by their chemical composition using techniques like spectroscopy.
- Impact on Corundum:
- Gemstone Value: Inclusions, in general, can decrease the clarity and, in some cases, the durability of gemstones, including corundum.
- Unique Optical Effects: Aligned mica inclusions within a sapphire can create a unique “mica-sheen” or “millefeuille” lamellar structure, distinct from the asterism of star sapphires.
- Geological Context: The presence and composition of mica inclusions can provide valuable information about the geological processes involved in the corundum’s formation and subsequent metamorphic history, according to GIA. Micas are often used for indirect dating of corundum deposits.
So far as doing futher analysis on your own, 3000 Euros is a lot of money, yikes!, I still think your best bet is to find someone interested in your rock at SFMC who will do more analysis without charging you money, just because their interested or because your sample will add to their research project. Bonne chance- good luck!
At least in my mind (I could just as easily be wrong), I think what Kadda has is a pegmatitic metamorphic sapphire. The minerological evidence already pointing that way.
Golden sheen sapphires as you already pointed out usually have iron minerals- hematite, goethite, magnetite, ilmenite creating the platelets and needles… iron mineral inclusions can be found in basaltic, metamorphic or both. Basaltic sapphires have more iron due to their residence in a mafic to syenitic silica undersaturated melt… The only reference that I could find was on basaltic sapphires in Southern Vietnam. Metamorphic sapphires from Mogok, Sri Lanka were metamorphosed in marbles. Ti/Fe ratios are higher.metamorphic sapphires than in basaltic, but only as a generalization. There’s too much overlap. The Fe oxidation state doesn’t really matter for color. Ti+4/Fe+3 can still exchange electrons at a higher energy band. Heat treatment which is pretty standard, oxidizes Fe+2 to Fe+3…FTIR spectroscopy shows the loss of two secondary absorptions bands around the primary band with heat. In general, basaltic sapphires are more oxidized than metamorphic ones.
The golden sheen sapphires from East Africa are considered metamorphic. The kicker is that I found one reference stating it was found in syenitic veins within the metamorphic host rock… that would also give it an igneous/ basaltic origin… Basaltic sapphires from China also have golden sheen. Black star sapphire deposits in Australia have a potential to find golden sheen ones.
I believe that you mentioned goetite as one of the iron minerals. That would be very interesting as goethite is thought of a secondary weathering mineral. If present as a primary constituent, if would mean that the environment of the formation of the sapphires was both highly oxidizing and hydrous. It could mean that there was retrograde metamorphism of other iron minerals (hematite hydration) by water and oxidation… Do you have a reference to goethite in corundum?
The overlap between basaltic and metamorphic corundums are broad. The petrogenesis of either of them is still under active research investigation, so far as I can determine from the literature. Basaltic corundum is of particular interest to me…Current theory is that pelitic high aluminum sediments (clay minerals) were carried by subducting slabs into deep crust or even as deep as lithospheric mantle… pooled and were tranformed into corundum… deep seated magma that was alkali, alkali silica undersaturated, like alkali basalt and lamprophyres (there’s a proliferation of rock name types allied to lamprophyres based on small differences in minerological and chemical compositions) bourght the sapphires to the surface in volcanic eruptions. These rocks are also allied to kimberlites which also suggest a deep origin. Corundum can be also be transported to the surface by kimberlites. I found a single paper that says that interaction with carbonatites at depth could also cause desilication. This issue is still under more research and debate. So far as the finest gems are concerned, Kashmir sapphires, now depleted were formed out of desilicated pegmatites. Mogok rubies and sapphires were layers of clay and lenses of clay, metamorphosed in a low silica rock- marbles (marbles are stable across almost the entire metamorphic grade spectrum).
You can see what I posted to Kadda… I hope he finds his answers without having to pay 3K euros…
Thanks for the tip off… I don’t know if you’re stil following this conversation. Kadda has something very interesting on his hands. The mica in corundum was found to be a calcium brittle mica- clintonite, margarite… also very interesting.
voir mon message à Troy aujourd’hui. Je crois que vous avez un rocher d’intresse plus scientifque que le valeur d’un bijou. Faites-nous savoir ce que vous comptez faire ensuite. Merci beaucoup et bonne journee.