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A variety of plagioclase feldspar which has a flashing display of varied colours suggestive of the Northern Lights. Although it has a dirty-grey colour before being cut, the cut stones, when placed in a certain position (but no other) before the viewer, change tremendously, acquiring a metallic lustre and a beautiful sheen, due to the lamellar composition and the property of adularescence, which in this stone is called 'labradorescence'.
The usual colour is blue, but a variety (called 'spectrolite') found in Karelia, Finland, shows the colour of the spectrum. The stones are usually cut with a flat surface, and have been carved as cameos. The original source, c. 1770, was the island of St. Paul, Labrador.
From: An Illustrated Dictionary of Jewelry, autor: Harold Newman, publishers: Thames and Hudson