The Collaborative International Dictionary of English v.0.48:

Glassy \Glass"y\, a.
   1. Made of glass; vitreous; as, a glassy substance. --Bacon.
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   2. Resembling glass in its properties, as in smoothness,
      brittleness, or transparency; as, a glassy stream; a
      glassy surface; the glassy deep.
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   3. Dull; wanting life or fire; lackluster; -- said of the
      eyes. "In his glassy eye." --Byron.
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   Glassy feldspar (Min.), a variety of orthoclase; sanidine.
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The Collaborative International Dictionary of English v.0.48:

Sanidine \San"i*dine\, n. [Gr. ?. ?, a board. So called in
   allusion to the tabular crystals.] (Min.)
   A variety of orthoclase feldspar common in certain eruptive
   rocks, as trachyte; -- called also glassy feldspar.
   [1913 Webster]