Labradorite

Labradorite is a calcium-rich plagioclase feldspar, a silicate mineral of the (Ca,Na)(Al,Si)₄O₈ series prized for the iridescent play of color known as labradorescence. Labradorite forms deep within the Earth's crust as magma slowly cools into igneous rock, primarily gabbro, basalt, and anorthosite. As the molten rock solidifies over thousands of years, alternating layers of different feldspar compositions crystallize in microscopically thin sheets, stacked like pages of a book.

This layered internal structure is what creates labradorescence, the spectacular flash of color that appears when light enters the stone and bounces between these internal planes. The thickness and spacing of the layers determine which wavelengths of light are reflected back, producing blues, golds, greens, and occasionally the full visible spectrum. It's essentially the same physics that make a soap bubble iridescent, but frozen in stone for hundreds of millions of years.

The finest specimens, known as spectrolite, come from Finland, where tectonic forces and specific cooling conditions created unusually regular internal layering that produces intense, full-spectrum color play.

Labradorite is most easily identified by its signature labradorescence, an iridescent play of color visible when the stone is rotated under light. The base color is typically dark gray to black, which distinguishes it from moonstone (which has a lighter base and a softer, more diffuse glow called adularescence).

At 6-6.5 on the Mohs scale, labradorite can be scratched by quartz but not by a steel knife. It has two directions of good cleavage at nearly right angles, and broken surfaces often show a pearly luster. The flash is directional. You may need to tilt the specimen to find the angle where color appears.