Iceland spar

plural Iceland spars

1. countableuncountableA transparent form of calcite, used to demonstrate the polarization of light.

   1 more example

   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.

   2006, Thomas Pynchon, Against the Day, Vintage, published 2007, page 127

   Synonyms: Iceland crystal