The pedestal of the famous «Bronze Horseman» unveiled in Saint Petersburg in August 7, 1782 was cut, as it is known, from the huge erratic boulder found at the coast of the Finnish gulf and called the «Thunder Stone». This boulder is always called rapakivi granite and identified with rocks of the Alexander Column, columns of the St. Isaac's Cathedral, stone plates of the Neva River embankment and numerous other monuments of old Saint Petersburg. Direct geological investigation of the «Thunder Stone» has been carried out by the authors in 2016 for the first time. Besides detail documentation it included mineralogical analysis of five spot samples, gamma spectrometry, etc. The rock is massive coarse-grained granite, somewhere with a trachytoid texture. Its principal mafic mineral is represented by biotite (annite). The overall composition of the granite is as following, % _vol : feldspar-81, quartz - 12, mafic minerals - 5. Accessory minerals are represented by rutile, zircon, monazite and thorite. There are also muscovite, topaz and an accessory thorium-uranium copper phosphate (not precisely defined) that are detected usually in pegmatites. The question of the place where the «Thunder Stone» was formed remains debatable.