Definitions

from The American Heritage® Dictionary of the English Language, 4th Edition

  • n. A glassy variety of orthoclase feldspar, known as moonstone when translucent.

from Wiktionary, Creative Commons Attribution/Share-Alike License

  • n. A variety of orthoclase feldspar found as tabular crystals embedded in volcanic rocks. It sometimes reflects a blue sheen along crystal planes. This phenomenon is called adularescence, and sanidine with this property is called moonstone.

from the GNU version of the Collaborative International Dictionary of English

  • n. A variety of orthoclase feldspar common in certain eruptive rocks, as trachyte; -- called also glassy feldspar.

from The Century Dictionary and Cyclopedia

  • n. A variety of orthoclase feldspar, occurring in glassy transparent crystals in lava, trachyte, and other volcanic rocks, chiefly those of comparatively recent age. It usually contains more or less soda.

Etymologies

Greek sanis, sanid-, board (from its flat crystals) + -ine2.
(American Heritage® Dictionary of the English Language, Fourth Edition)

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