Evaluation of dimension stone in gneissic rocks — a case history from southern Finland

A dimension stone prospect in southern central Finland was assessed by a detailed mapping, geo-radar survey, and core drilling. The prospect is a veined and bedded garnet–cordierite gneiss, consisting of a dark schistose medium-grained palaeosome and a light coarse-grained granitic leucosome. Both components are found mainly as thin units, but the leucosome can occur as individual veins several metres thick, which leads to disturbing variations in the appearance of the stone. The soundness of the stone is defined by the amount of tight, but open cracks in the palaeosome and weakness zones in the weathered leucosome. As a whole, the soundness is diminished down to 10 m below the outcrop surface. Furthermore, a subhorizontal body of younger granite is identified at approx. 8–20 m depth. Consequently, the prospect is not feasible for production of dimension stone. re drilling was decisive in the final evaluation of the prospect because the true density of the cracks and the subsurface granite were identified by the method. Only with the use of core drilling could the proper interpretation of the characteristics of the prospect be made, demonstrating the importance of three-dimensional investigation of a dimension stone prospect.