Porphyry is extracted in the upper Val Brembana. From a petrographic point of view, it is a meta limestone consisting of silicates which account for its excellent technical characteristics. It features high values of mechanical strength along with significant values of bending strength, frost and wear resistance. All this makes it an ideal material for outdoor use. The low values of wearability and alterability translate into outstanding durability, to the extent that the laid material requires no maintenance.
An important property is its marked susceptibility to naturally split into thin sheets. The so-called cleavage results from its having been subjected to considerable compression efforts associated with the formation of the Alps. The presence of planar structures that are the preferential cleavage planes allows the splitting of the stone in order to obtain thin sheets. The typical use of porphyroid is roof covering, in which its valuable aesthetic characteristics are conjugated with its excellent structural and mechanical properties. The slabs (called "piode") are available in several sizes and they can be utilised in various laying styles as a function of the inclination of the roof, climatic conditions and construction techniques.
The low wearability of the material makes it particularly appropriate for flooring, not only for pedestrian areas, but also for high traffic zones. There are two varieties.
Dark Gray Porphyry (also known as Valleve Slate) — Derived from Triassic rocks (225 million years) belonging to the formation of Wengen. The typical characteristic of dark gray porphyry, compared with the rest of the Wengen formation outcropping in the Grigne mountains, in Val Scalve, or in the Dolomites, is its silicates rich mineralogical composition.
Gray Porphyroid (also known as Branzi slate) — It is extracted in outcrops of the Collio formation that dates back to the Permian period (roughly 290 to 280 million years ago) and derives from ancient river and lake deposits, i.e. sand and mud, which in time consolidated to become rock. It shows gray-blue hues ranging from light to dark tones.