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COLOUR

Porphyry is never evenly coloured. The natural cleft surface of both rough and processed elements is disseminated with chromatic variations and stripes or spots of different colours in a more or less compact matrix, although one hue usually predominates over others. Colour depends on the rock's chemical composition, namely on the main and accessory mineral constituents. The main constituents, primarily responsible for the varied colouring of porphyritic rock, are quarz and several feldspars (plagioclase, orthoclase, microcline), while the accessory constituents are usually mafic minerals (biotite, hornblende, several pyroxenes) composed predominantly of ferromagnesian rock-form-ing silicates. Seeing that the variety of colours of porphyrytic rocks coming from the various quarries is so wide and complex, it is impossible to give a clear-cut classification of porphyry colours. To simplify, we will refer to the two principal, basic colour ranges which can be found on the market - grey and red - although the ranges comprise a variety of colours. The natural cleft porphyry in Fig. 6 shows the various shades which the mixed grey range takes on. It is never uniform but the dominant hues are grey, purple-grey and light brown. In natural cleft porphyry's red range further classification is possible, according to the intensity of the predominant red tones and of the surface grain. This classification gives us rossiccio misto (mixedreddish) (Fig. 7), rosso marrone (reddishbrown) (Fig. 8) and rosso scelto (choicered) (Fig. 9). Porphyry with a finished surface is never uniform in colour either and in most cases shows a predominance of grey, brown and red hues. In this case, three types of colour shadings may be identified, which can be traced to the two grey ranges: Stardust grey® (Fig. 10) and marrone-grigio (brownish-grey) (Fig. 11); and to the red range: rossiccio misto (mixed reddish) (Fig. 12).