Cordierite is a ferromagnesian aluminous silicate. It is a mineral of the Contact and Regional (Low Pressures) metamorphisms, formed in clay rocks; but it is also present in granites derived from the melting of the continental crust (such as that of the Velay Massif ).

        In this Gneiss of the Ihosy district (Madagascar), Cordierite, in large, centimetric gem crystals, navy blue in colour, is exploited under the name of "Water Sapphire". It is associated with Feldspar and Quartz (white), Biotite (small black crystals), Garnet (difficult to distinguish) and microscopic Sillimanite.

Under the microscope, cordierite is difficult to distinguish from quartz when unaltered. In the microphotograph below, it has a slightly dusty appearance. The identification is helped by the presence of a yellow-orange halo in the cordierite around small crystals of zircon, monazite and sometimes apatite.

This photo under plane-polarised light shows cordierite, quartz, sillimanite and "opaque" minerals (ilmenite and magnetite).

In the Velay Massif, on the south-eastern side of the Massif Cental, anatexites and granites, the ultimate terms of a LP-HT metamorphic gradient, contain centimetric green crystals of cordierite:

Under the microscope, a beginning of alteration helps to recognize the mineral:

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