The strength of a rock excluding weaknesses due to fractures and joints. It is measured by the rebound value in a Schmidt hammer test and by its response to a blow from an ordinary hammer. A very strong rock such as quartzite, dolerite, or basalt will break only under severe blows from the hammer and has a rebound value of 100–60. A hammer blow will break a hand-held specimen of a strong rock (60–50) such as marble, granite, or gneiss. A firm blow will make a shallow indentation on the surface of a moderately strong (50–40) rock such as sandstone or slate and a deep indentation on a weak (40–35) rock such as coal or schist. A very weak (35–10) rock such as chalk or halite can be cut with a knife and a sharp hammer blow will make it crumble.
Subjects: Earth Sciences and Geography.