labradorescence

Labradorescence, also spelled labradorescence, is a striking iridescent optical effect observed in many specimens of labradorite, a plagioclase feldspar. The phenomenon appears as flashes of color that shimmer and shift as the stone is moved, often showing blues and greens with occasional golds, oranges, or purples. The effect is highly prized in jewelry and decorative carving.

Cause and mechanism: Labradorescence arises from the internal crystal structure of labradorite. During crystallization, lamellae of differing compositions exsolve to form finely layered microstructures within the mineral. These lamellae, typically a few hundred nanometers thick, cause light entering the crystal to reflect and interfere between successive layers. The result is constructive and destructive interference that produces the observed color play. The colors and intensity depend on the thickness and spacing of the lamellae and the angle of viewing and illumination.

Occurrence and terminology: The term is named after Labrador, Canada, where high-quality specimens were first described. A related, highly vivid form from Finland is marketed as spectrolite. Not all labradorite exhibits strong labradorescence; the effect varies among specimens due to differences in internal lamellar structures.

Relation to other phenomena: Labradorescence is a type of iridescence or schiller. It is distinct from adularescence, the bluish sheen seen in moonstone, though both are depth-related optical effects in feldspar minerals.

Applications: Labradorite displaying strong labradorescence is commonly cut as cabochons or polished for use in jewelry and ornaments.