Corundum is one of the most valued gem stones occurring naturally in a variety of geological settings. Favourable settings for the crystallization of gem quality corundum are: (1) Corundum mineralization associated with metasomatic/metamorphic processes in high grade rocks (interaction gneiss/pegmatite with mafic/ultramafic rocks); (2) corundum from marbles, (3) corundum (sapphire) occurring as xenocrysts in volcanic rocks; and (4) corundum from pegmatite-like rocks in a feldspathic matrix. Another relatively unusual occurrence of gem quality corundum bearing pyroxenitic xenoliths in pliocene and pleistocene basalts from SE-Thailand. The geological process of formation of these primary deposits are of gemmological, economic and scientific interest. They are responsible for the development of characteristic diagnostic features such as growth features, mineral and fluid inclusions, colour, brightness as well as chemical and stable isotopic composition. Well known occurrences of ruby and sapphire are Sri Lanka, India, Myanmar, Cambodia, Thailand, Vietnam, Australia, Kenya, Tanzania, and Madagascar.

The amount of trace elements in corundum can vary considerably within the same deposit. The differences in chemical compositions of corundum from one deposit type to another are sometimes smaller than the variation within the same locality. Therefore it is difficult to assign a typical trace element pattern of ruby and/or sapphire to a specific deposit. Nevertheless some elements may be useful for deducing a possible geological setting of formation, e.g. Ni is most likely found in corundum related to ultramafic to mafic rocks. A completely different picture is given by the oxygen isotope composition. The geological setting determines the isotopic compositions: (1) corundum from ultramafic xenolith have ∂¹⁸O of 2-3 ‰. Corundum xenoliths in basalts have an isotopic signature in the range of typical basaltic rocks, which is between 4 and 6 ‰. Corundum from amphibolites, where a gabbro or basalt is the precursor rock also have an isotopic composition around 5-6 ‰. Rubies of metamorphic and/or metasomatic origin have ∂¹⁸O of ~12-14 ‰. Rubies and sapphires which are associated to marbles have a typical sedimentary signature with ∂¹⁸O of 18-24 ‰. The lower values probably indicate some interaction with a metamorphic fluid. This assumption is very likely since all these marble hosted deposits are within a granulite-facies crystalline basement.

Brown amphibole is found only in corundum bearing marble. Within a matrix of coarse grained calcite 5 -20 mm large prismatic amphibole crystals occur. In addition, centimeter large corundum crystals, some graphite and sulfides are found. The chemical composition of the brownish amphibole is aluminum and fluorine rich and can be classified as fluorian Aluminopargasite to fluorian Magnesiosadanagaite. The second type of amphibole is usually associated with red spinel and is found in coarse grained calcite-dolomite marbles. However, in some samples green amphibole also occurs without spinel. The crystals can measure up to 5 cm, are idiomorphic with an intensive green colour and good luster. These amphibole crystals contain less aluminum compared to the first type, are also rich in fluorine, contain some Cr and can be classified as fluorian Pargasite.

A cordierite sample (42/IA) from Kiranur, South India, and an ilvaite sample (ISX1) from Serifos, Greece, have been characterized and prepared for use as potential electron microprobe reference samples. By combining different high precision analytical methods we determined the major, minor and trace element composition of these samples and proved their near-stoichiometric composition and high homogeneity at the micron level. The chemical composition (working values) of ilvaite ISX1 is SiO₂: 29.68%, Al₂O₃: 0.67%, Fe₂O₃: 34.72%, FeO: 18.67%, FeO_tot: 51.52 %, MnO: 0.31%, MgO: 0.22%, CaO: 13.76%, H₂O: 2.21%, and of cordierite 42/IA is SiO₂: 49.65%, Al₂O₃: 33.17%, FeO_tot: 2.34%, MnO: 0.03%, MgO: 12.44%, Na₂O: 0.33%, H₂O: 1.21%, CO₂: 0.59%. Aliquots of these samples for use as primary or secondary electron microprobe reference sample can be obtained on request.