Natural stone defined as trachyte-trachyandesite from the locality Colli Euganei near Padua in Italy is built into roadway-pedestrian construction of the Tkalčićcva street in Zagreb. Stone samples were subjected before in-building, to determinations of physical and mecha¬nical properties (density, bulk density, water absorption, porosity, compressive strength, resistance to abrasion, flexural strength, frost resistance and to Na2SO4, velocity of ultrasonic propagation, micros¬copic and X ray analyses and to photographing by electronic micros¬cope. It was found that about 30% in-built stones do not suit to existing criteria on the quality of natural stone. Water absorption and porosity are increased, compressive strength in the water saturated condition is very decreased, the stone is not enough resistant to wear-out and to Na2S04, it is anisotropic and contains swelling clays of the smectite group and numerous defects especially in form of microblocks as consequences of postdiagenetic processes. Although the samples belong to the same rock type, the differences among stone elements are great. Macroscopically materia! is very heterogeneous. In less qualitative stone samples there are more or less numerous sub-parallel brown and gray partings of the thickness approximately 1 -2 mm with enlarged share of secondary iron minerals. Iron oxides and hydroxides mainly impregnate the rock parts, spotty or stripy; somewhere the complete sample is impregnated intensively. The impregnation is intensified by swelling clays and numerous holes along which stone is crumbling.