Caesium

Caesium is a very soft, pale-gold alkali metal that melts at about 29°C, so it can become liquid in a warm room. Like other alkali metals, it reacts readily; contact with water causes a violent reaction. In nature, it occurs in compounds rather than as free metal. Its main ore is pollucite, a mineral found in coarse-grained rocks called pegmatites. Processing the ore yields caesium salts, which can be converted into the metal by chemical reactions. The stable isotope caesium-133 provides the reference frequency for caesium atomic clocks and the definition of the second. A different use involves the compound caesium formate: its dense solutions help control pressure while drilling oil wells.