Six commercial types of natural stone of the Madeira Archipelago were submitted to standard physico-mechanical tests, namely: mechanical resistance to compression, and mechanical resistance to flexion, before and after freezing-thawing, apparent volumetric weight, absorption of water at normal pressure conditions, open (or apparent) porosity, thermal linear expansion coefficient, abrasion test and impact test. The research carried out did show the dependence of the physico-mechanical behaviour on some lithological and textural characteristics of the natural stone.
In national terms, natural stone represents a natural resource economically of paramount importance that requires thorough scientific and technological studies (Moura, 1991, Costa et al., 1995). In regional terms, in what concerns the Madeira archipelago, the extractive, manufacturing and commercial activities using natural stone as raw material have great social, cultural and economic expression, higher in Madeira island than in Porto Santo island. However, presently, in the Madeira archipelago, and in commercial terms, the demand of natural stone is higher than the offer of natural stone. The study of the main physical, chemical and technological characteristics of the natural stone herein presented pretends to be a contribution for the use of this resource in civil construction works, concerned with the its quality and reserves. Lavic and pyroclastic rocks quite common in the Madeira archipelago have been used since the begin of the settlement (1420 – 1425) in the construction of houses, monuments and domestic tools, and recently as material for modern art works. The present work deals with the study of the most representative, both in qualitative and quantitative terms, lithological types of natural stone. It should be emphasized that any lithological and textural heterogeneities of the natural stone, namely the inclusions or xenotiths, act as penalising factors devaluating its commercial value. The commercial nomenclature used in the Madeira archipelago for the characterisation of natural stone considers just two nominations, based on stone hardness, cohesion, resistance to cut, carving or sawing, and colour: "cantaria rija", i.e., hard stone, and "cantaria mole", i.e., soft stone.
Most of the volcanic rocks that are being utilized as natural stone in the Madeira island, can be grouped into two distinct sets: one consisting of lavic rocks of basaltic composition (s.l.), either compact or more or less vacuolar; the other consisting of pyroclastic rocks, more abundant in the island inland, exhibiting distinct textural types, from great blocs down to fine ashes, including intermediate grain sizes, generally under the form of lapilli tuff, more or less consolidated (Carvalho & Brandao, 1991). In Porto Santo island natural stone is extracted in two distinct sites, one located in the NE of the island consisting of effusive lavic rocks of intermediate to acid composition, essentially trachyte, the other located in the SW of the island consisting of pyroclastic rock, essentially lapilli tuff. Natural stone is being extracted as well, on a temporary basis, from the big blocs which have fallen down onto the bed of short rivers, generally made up of lapilli tuff.