Labradorite is part of the plagioclase feldspar family, with a chemical composition of (Ca,Na)(Al,Si)₄O₈. It’s made up of calcium and sodium aluminosilicates, along with small amounts of potassium feldspar. The balance of these elements in its crystal structure directly influences its optical effect, also known as labradoresence.

Within labradorite, there are thin, parallel layers known as twinning planes. These are key to its colourful, flashy appearance. Twinning planes form when the crystal grows in such a way that one part mirrors the other. In labradorite, these planes are exceptionally fine and numerous, creating the foundation for its famous optical effect.

So, what exactly is labradorescence? It’s the term for the vibrant flashes of colour that labradorite is known for. This iridescent effect happens because of light interacting with the crystal’s twinning planes.

When light enters the stone, it hits these planes and scatters. As the light waves interfere with each other, they produce the vivid colours that define labradorite. The colours shift depending on the angle of light and the way you view the stone, giving it a dynamic appearance.

Causes of Iridescence

Iridescence (including labradorescence) happens when light bends (diffraction) or reflects off thin layers inside a material. In labradorite, both processes work together to create its colourful flashes:

• Diffraction of Light: The thin, parallel layers inside labradorite act like tiny slits (a diffraction grating), splitting white light into a rainbow of colours. It’s similar to the way light creates a rainbow on a CD or in a soap bubble.

• Thin Film Interference: When light hits the thin layers, part of it reflects off the top layer while some reflects off the bottom. These reflections overlap, either brightening colours when the waves match up (in phase) or dimming them when the waves cancel each other out (out of phase).

Colour Ranges

Labradorescence can produce a wide range of colours depending on the thickness and spacing of the twinning planes:

• Blues and Greens: These are the most common and vibrant colours seen in labradorite. They result from thinner twinning planes that scatter light more effectively.

• Gold and Yellow: Less common but highly sought after, these colours are produced by slightly thicker twinning planes.

• Orange and Red: Rare and highly desirable, these colours appear in very high-quality specimens and result from even thicker twinning planes.

• Purple and Pink: Extremely rare, these colours occur under specific conditions within the crystal structure and are usually only seen in the finest specimens.