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Cabo Ortegal is one of the major ultramafic complexes outcropping in Iberia. Most rocks are variably serpentinized, and the characteristics of the process can be traced through the composition of the rocks. The major and trace elements change in the different units of the complex. Together with the wide range of oxygen isotope values (d 18 O from 3.9 up to 6.6‰), these changes lead to the conclusion that several stages of serpentinization took place at Cabo Ortegal. Each stage is characterized by the composition of the fluid that produced the serpentinization. Seawater does not have to be the main fluid in the process; an ultimate stage of serpentinization can mask the characteristics of a previous one.

Many questions arise when considering the appropriate use of building stones such as serpentinites. The commercial names of these rocks, collectively grouped as 'Green marbles', have no correspondence to their actual mineralogy, geochemistry and/or physical characteristics. Serpentinite being the hydrated product of an ultramafic parent rock and not a metamorphic product of limestone as implied by the term 'marble'. However, the serpentinites most widely used for ornamental purposes come from India (e.g. Rajasthan Green, Emerald Green) and in these the original mineralogy has been almost completely converted into carbonates. By contrast, serpenfinites from elsewhere (e.g. Vermont Verde Antique from the USA and Verde Pirineos from Spain) do preserve some of their original mineralogy. The different physical and chemical behaviour of carbonates and serpentine minerals can result in significantly different behaviour of commercial building stones. Thus, carbonates are resistant to weathering but suffer from acidic cleaning agents in interior use; whereas serpentinites, with a high content of talc, used on external faces undergo an increase in volume and a consequent rapid degradation.Accurate and precise characterization of serpentinites, including information on their mineralogy and geochemistry (including major, trace and volatile elements together with oxygen isotopes), in conjunction with their physical properties, would enable architects to select the appropriate interior or exterior use of these handsome building stones.

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