The Collaborative International Dictionary of English v.0.48:

lavaliere \la`va`liere"\, lavallier \la`val`lier"\, or La
valliere \La val`liere"\, n. [after the Duchesse de La Valliere,
   a mistress of Louis XI. --RHUD.]
   A neck ornament consisting of a chain and single jewelled
   pendant, or drop; also, the pendant itself.
   [Webster 1913 Suppl. +PJC]

WordNet (r) 3.0 (2006):

lavaliere
    n 1: jeweled pendant worn on a chain around the neck [syn:
         lavaliere, lavalier, lavalliere]