The Collaborative International Dictionary of English v.0.48:

Turquoise \Tur*quoise"\, Turquois \Tur*quois"\ (?; 277), n. [F.
   turquoise; cf. Pr. & Sp. turquesa, It. turchese, turchina,
   LL. turchesius, turchina; -- so called because first brought
   from Turkey. See 1st Turkey.] (Min.)
   A hydrous phosphate of alumina containing a little copper;
   calaite. It has a blue, or bluish green, color, and usually
   occurs in reniform masses with a botryoidal surface.
   [Formerly written also turcois, and turkois.]
   [1913 Webster]

   Note: Turquoise is susceptible of a high polish, and when of
         a bright blue color is much esteemed as a gem. The
         finest specimens come from Persia. It is also found in
         New Mexico and Arizona, and is regarded as identical
         with the chalchihuitl of the Mexicans.
         [1913 Webster]