Quantum phase transition in the dioptase magnetic lattice
  The study of quantum phase transitions, which are zero-temperature phase transitions between distinct states of matter, is of current interest in research since it allows for a description of low-temperature properties based on universal relations. Here we show that the crystal green dioptase Cu_6Si_6O_18 . 6H_2O, known to the ancient Roman as the gem of Venus, has a magnetic crystal structure, formed by the Cu(II) ions, which allows for a quantum phase transition between an antiferromagnetically ordered state and a quantum spin liquid. 
C. Gros, P. Lemmens, K.-Y. Choi, G. G"untherodt, M. Baenitz and H.H.   Otto