Source : Webster's Revised Unabridged Dictionary (1913)

Water sapphire \Wa"ter sap"phire\ [Equiv. to F. saphir d'eau.]
   (Min.)
   A deep blue variety of iolite, sometimes used as a gem; --
   called also {saphir d'eau}.

Source : WordNet®

water sapphire
     n : a deep blue cordierite often used as a gemstone