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Photomicrography > Corundum > Magmatic rubies
First draft: May 15^th, 2014 | Last update: April 4^th, 2017

Some Features Observed in Magmatic Rubies & Sapphires
Australia, Cambodia, southern China, Kenya, Laos, northern Madagascar, Thailand & southern Vietnam

The inclusion scene observed in magmatic corundums is completely different from that of their metamorphic counterparts. Consequently, microscopic observation generally renders it possible to distinguish between both families. Yet, magmatic sapphires do not share the same genesis as magmatic rubies.

Magmatic sapphires crystallised from (partially) melted mantle material, while magmatic rubies formed due to metamorphism of mafic to ultramafic rocks at great depths and thus under substantially higher pressures and temperatures than in metamorphic environments, resulting in a particular inclusion scene.

References and items of related interest

Coenraads, R.R. (1992). Surface features on natural rubies and sapphires derived from volcanic provinces. The Journal of Gemmology, Vol. 23, No. 3, pp. 151-160.

Coenraads, R.R. (1992a). Sapphires and rubies associated with volcanic provinces: inclusions and surface features shed light on their origin. The Australian Gemmologist, August, pp. 70-78.

Coenraads, R.R., Sutherland, F.L. & Kinny, P.D. (1990). The Origin of sapphires: U-Pb dating of zircon inclusions sheds new light. Mineralogical Magazine, Vol. 54. pp. 113-22. Mineralogical Magazine Tables of Contents

Collins, A.S. (2000). The tectonic evolution of Madagascar: Its place in the East African Orogen. Gondwana Research, Vol. 3, No. 4, pp. 549–552.

Garnier, V., Giuliani, G., Ohnenstetter, D. & Schwarz, D. (2004). Saphirs & Rubis, Les gisements de corindon: classification et genese. Le Regne Mineral, No. 55, 47 p (in French).

Garnier, V., Ohnenstetter, D., Giuliani, G. & Schwarz, D. (2003). Revue des classifications des gisements de corindon. Chapitre 4, Ecole Doctorale RP2E, France (in French).

Giuliani, G., Ohnenstetter, D., Fallick, A.E., Groat, L. & Fagan, A.J. (2014). The geology and genesis of gem corundum deposits In: Groat, L.A. (Editor) Geology of gem deposits, pp. 29-112. Mineralogical Association of Canada, Short Course Series, Vol. 44, Quebec, Quebec, Canada, 405 p. ISBN 978-0-921294-54-2.

Gübelin, E.J. (1983). The Precious Corundums Ruby and Sapphire In: Internal World of Gemstones. Documents from Space and Time, pp. 112-136. 3^rd edition. ABC Edition, Zürich, Switzerland, 234 p. ISBN 3-85504-030-3.
Note: 1^st Edition (1973). Innenwelt der Edelsteine. Urkunde aus Raum und Zeit. ABC Edition, Zürich, Switzerland, 233 p. ISBN 3-85504-030-3 (in German; ISBN checked in the original copies).

Gübelin, E.J. & Koivula, J.I. (1997). Ruby and Sapphire and their Inclusions In: Photoatlas of Inclusions in Gemstones, pp. 324-361. 3^rd revised edition. ABC Edition, Zürich, Switzerland, 532 p., ISBN 3-85504-095-8.

Gübelin, E.J. & Koivula, J.I. (2008). Inclusions in Major Commercial Gems: Ruby and Sapphire In: Photoatlas of Inclusions in Gemstones. Volume 3, pp. 53-353. Opinio Publishers, Basel, Switzerland, 672 p., ISBN 3-03999-029-2.

Guo, J.F., O'Reilly, S.Y. & Griffin, W.L. (1996). Corundum from basaltic terrains: an inclusion approach to the enigma. Contributions to Mineralogy and Petrology, Vol. 122, Issue 4 (January), pp. 368-386. abstract

Groat, L.A. (2014). Geology of Gem Deposits. 2^nd Edition. Short Course Series, Vol. 44. Mineralogical Association of Canada, Québec, Québec, Canada, 405 p. ISBN 978-0-921294-54-2. Mineralogical Association of Canada

Handke, M.J., Tucker, R.D. & Ashwal, L. (1999). Neoproterozoic continental arc magmatism in west-central Madagascar. Geology, Vol. 27, No. 4, pp. 351–354. abstract

Hänni, H.A. (1994). Origin determination for gemstones: possibilities, restrictions and reliability. The Journal of Gemmology, Vol. 24, No. 3, pp. 139-148. Also published on www.ssef.ch

Kievlenko, E.Y. (2003). Geology of Gems. English Ed., Ocean Pictures Ltd., Littleton, CO, USA, 432 p. + 32 p. plates. ISBN 978-5900395258.

Levinson, A.A. & Cook, F.A. (1994). Gem Corundum in Alkali Basalt Origin and Occurrence. Gems & Gemology, Vol. 30, No. 4, pp. 253-262.

Pain, C.F. (1983). Geomorphology of the Barrington Tops area, New South Wales. The Journal of the Geological Society of Australia, Vol. 30, pp. 187-94. abstract

Peucat, J.-J., Ruffault, P., Fritsch, E., Simonet, C., Bouhnik-Le Coz, M. & Lasnier, B. (2005). Un nouvel outil géochimique de reconnaissance des saphirs bleus basaltiques et métamorphiques: le rapport Ga/Mg. Revue de Gemmologie, Association Française de Gemmologie, No. 153, pp. 8-12 (in French).

Schwarz, D. (1998). Aus Basalten, Marmoren und Pegmatiten. Spezielle Ursachen form ten in der Erdkruste edle Rubine und Saphire In: Glas, M. (Editor) Rubin, Saphir, Korund: schan, hart, selten, kostbar. extraLapis, No. 15, pp. 5-9. Christian Weise Verlag, München, Germany, 96 p. ISBN 9-921656-45-1 (in German).

Simonet, C. (1997). Géologie des Gisements de Saphirs. Diplôme d'Université de Gemmologie (DUG) Dissertation, University of Nantes, 82 p. (in French). Published on www.gemnantes.fr

Silva, K.K.M.W. & Siriwardena, C.H.E.R. (1988). Geology and the origin of the corundum-bearing skarn at Bakamuna, Sri Lanka. Mineralium Deposita, Vol. 23, Issue 3 (July), pp. 186-190. abstract + references

Sorkhabi, R.B. & Stump, E. (1993). Rise of the Himalaya: A geochronologic approach. GSA Today, Vol. 3, No. 4, pp. 85, 88-92.

Sutherland, F.L. & Schwarz, D. (2001). Origin of gem corundums from basaltic fields. The Australian Gemmologist, First Quarter, Vol. 21, pp. 30-33.

Sutherland, F.L. & Schwarz, D. (2003). Origin of gem corundums. The Journal of Gemmology, Vol. 28, No. 8, pp. 473-452.

Sutherland, F.L., Hoskin, P.W.O., Fanning, C.M. & Coenraads, R.R. (1998a). Models of corundum origin from alkali basaltic terrains: a reappraisal. Contributions to Mineralogy and Petrology, Vol. 133, Issue 4 (December), pp. 356-372. abstract

Sutherland, F.L., Schwarz, D., Jobbins, E.A., Coenraads, R.R. & Webb, G. (1998b). Distinctive gem corundum suites from discrete basalt fields, a comparative study of Barrington, Australia, and West Pailin, Cambodia, gem fields. The Journal of Gemmology, Vol. 26, No. 2, pp. 65-85.

Wade, S. (2002). Origin blues. Colored Stone, Vol. 15, No. 3, pp. 38-39.

Zaw, K., Sutherland, L., Graham, I. & McGee, B. (2008). Dating zircon inclusions in gem corundums from placer deposits, as a guide to their origin. 33^rd International Geological Congress, Oslo, August 6-14^th. abstract

More corundum references

Photomicrographs of Magmatic Rubies

Legend: FoV: Field-of-View; DoF: Total focusing depth; DF: darkfield; Obl: oblique illumination (fiber optics); Pol: polarising filter before objective; Sh: shadowing; TM: transmitted illumination, UVL: long-wave ultra-violet radiation; UVS: short-wave ultra-violet radiation; X-Pol: crossed polarising filters

Polysynthetic twinning
40x (FoV ± 3.5mm) X-Pol

Polysynthetic twinning
50x (FoV ± 2.5mm) X-Pol

Polysynthetic twinning
25x (FoV ± 4mm) X-Pol

Polysynthetic twinning along rhombohedral faces in a 1.25ct natural (magmatic) ruby
18x (FoV ± 4mm) X-Pol

Surface reaching (a bit hard to see) twin planes observed in magmatic ruby
18x (FoV ± 3mm) X-Pol

Surface reaching twinning plane
30x (FoV ± 3.5mm) X-Pol

Negative crystal & veil with two-phase filling
50x (FoV ± 2.5mm) Obl

Boehmite, negative crystal & veils
30x (FoV ± 3.5mm) Obl, Sh

2-phase negative crystals are common
50x (FoV ± 2.5mm) Obl, Sh, Pol

Negative crystal intersected by twin planes
50x (FoV ± 2.5mm) Obl, Sh, Pol

Minute veils of fluid inclusions
50x (FoV ± 2.5mm) DF, Pol

Rosettes
50x (FoV ± 2.5mm) Obl, Pol

Pyrrhotite crystal & veils
30x (FoV ± 3.5mm) Obl, Sh, Pol

Pyrrhotite and negative crystals are always truncated
50x (FoV ± 2.5mm) Obl, Pol

Pyrrhotite and its archetypal surrounding fingerprint - gem courtesy of J.C. Höfelt
40x (FoV ± 3mm) Obl, Pol

Magmatic corundums often contain negative crystals with glassy residues
50x (FoV ± 2.5mm) Obl, Pol

Fluid inclusions in a 1.22ct ruby
50x (FoV ± 2.5mm) Obl, Pol

Boehmite needles along twinning planes
35x (FoV ± 3.5mm) Obl, Pol

Boehmite needles with fringes
50x (FoV ± 2mm) Obl, Pol

Boehmite along 3D intersecting twin lamellae emphasises the rhombohedron and is diagnostic for magmatic rubies
30x (FoV ± 4mm) Obl, Pol

Boehmite needles and negative 2-phase inclusions
30x (FoV ± 2.5mm) Obl, Pol

Very often, identifying inclusions is quite problematic. Protogenetic inclusions are mostly heavily worn or damaged, lack crystal habit, and prevent identification while heat treatment provokes discoloration of inclusions and formation of stress haloes. Such problems calling for analysis are presented in the following photomicrograph of a 0.77ct which intuitively would indicate a magmatic origin.

Mind games of a ruby: uranium pyrochlore, rutile, garnet... or perhaps something else?
25x (FoV ± 3mm) Obl, Pol

The inclusion may be a strong indication for a heat-treated stone in which a pre-existing inclusion has been altered causing the surrounding tension halo.

But the appearance of this particular scene is strikingly similar to uranium pyrochlore inclusions found in magmatic sapphires which may stir up confusion.

A case like this illustrates the point where gem labs are usually called in. Non-destructive analysis of the inclusion and of the stone are necessary in order to provide us with clear answers.

Photomicrographs of Magmatic Sapphires

Legend: FoV: Field-of-View; DoF: Total focusing depth; DF: darkfield; Obl: oblique illumination (fiber optics); Pol: polarising filter before objective; Sh: shadowing; TM: transmitted illumination, UVL: long-wave ultra-violet radiation; UVS: short-wave ultra-violet radiation; X-Pol: crossed polarising filters

Extremely conspicuous boehmite inclusions with iron oxides (?) & fluid inclusions
18x (FoV ± 3.5mm) X-Pol, Fiber illumination

The boehmite inclusions are aligned along the twin planes - note the interference colours
18x (FoV ± 3.5mm) X-Pol, Fiber illumination

Boehmite and small aligned "fingerprints": a detail
50x (FoV ± 1.7mm) X-Pol, Fiber ill.

Such fingerprints, such as the left part of this example, are diagnostic for the origin of this 2.83ct untreated sapphire
30x (FoV ± 2.0mm) Fiber illumination, Pol

(Almost) a full house: crystals (plagioclase ?) with haloes, a comet (zircon ?), and liquid feathers
50x (FoV ± 2.0mm) Fiber illumination, X-Pol

A more detailed recording of one of the supposedly plagioclase crystals
50x (FoV ± 1.0mm) Fiber illumination, X-Pol

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