 Divalent europium ( Eu2 + ) in small amounts is the activator of the bright blue fluorescence of some samples of the mineral fluorite ( CaF2 ) . The reduction from Eu3 + to Eu2 + is induced by irradiation with energetic particles . The most outstanding examples of this originated around Weardale and adjacent parts of northern England ; it was the fluorite found here that fluorescence was named after in 1852 , although it was not until much later that europium was determined to be the cause . 
