  Cesium formate powder. 
Cesium formate, dissolved or suspended in water, is used as a drilling fluid for drilling deep oil wells.  These solutions are extremely dense, allowing them to "float" rock chips and dust to the surface from the bottom of a deep well as it is being drilled.   
The "purity" figure below, 75%, refers to the weight percent of cesium in cesium formate.  Because cesium is such a heavy atom, it makes up most of the weight of the compound even though the formate group contains four other atoms. 
I am extremely grateful to the Cabot Corporation for very kindly sending me a bottle full of this rather expensive compound, as well as jugs of cesium formate brine and suspension. 
Source: Cabot Corporation 
Contributor: Cabot Corporation 
Acquired: 28 March, 2009 
Text Updated: 29 March, 2009 
Price: Donated 
Size: 2" 
Purity: 75%
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