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Iceland spar (countable and uncountable, plural Iceland spars)

  1. A transparent form of calcite, used to demonstrate the polarisation of light.
    • 2006, Thomas Pynchon, Against the Day, Vintage 2007, p. 127:
      portions of the original aragonite—which make up the nacreous layers of the pearl—had, through “induced paramorphism,” as it was known to the artful sons of Nippon, been selectively changed here and there to a different form of calcium carbonate—namely, to microscopic crystals of the doubly refracting calcite known as Iceland spar.