Labradorite is a naturally occurring mineral formed within the Earth’s crust, not a man-made substance. The question about its origin often arises because of its stunning visual effect, which appears almost artificial. This characteristic phenomenon, known as labradorescence, involves a vibrant, shifting flash of color. Despite its striking appearance, the mineral’s composition and structure are entirely the result of geological processes.
A Natural Mineral: Composition and Classification
Labradorite is part of the extensive feldspar group. Specifically, it belongs to the plagioclase feldspar series, a continuous sequence of minerals defined by the relative proportions of calcium and sodium. The chemical composition of this mineral is defined as a calcium-sodium aluminum silicate, represented by the formula \((\text{Ca},\text{Na})(\text{Al},\text{Si})_4\text{O}_8\).
This composition places it in an intermediate range between the sodium-rich endmember (albite) and the calcium-rich endmember (anorthite). For a mineral to be classified as labradorite, its anorthite content must be between 50% and 70%. Its internal structure is triclinic, meaning its atomic lattice is highly complex with three unequal axes that meet at non-90-degree angles.
The Science Behind Labradorescence
Labradorescence is not a pigment or surface color but a physical interaction of light with the stone’s internal structure. This phenomenon is caused by the interference of light waves reflecting off microscopic, layered structures within the mineral. These layers, known as exsolution lamellae, formed when the mineral cooled slowly, causing calcium and sodium components to separate into distinct, alternating layers.
These thin layers, often less than a thousandth of a millimeter thick, act like a diffraction grating. When white light enters the stone, it strikes these parallel layers and is partially reflected off each boundary.
The light waves reflecting from different layers travel slightly different distances before exiting the stone and recombining. When these waves recombine, some wavelengths of light cancel each other out while others reinforce each other, a process called thin-film interference. The thickness of the lamellae determines which specific colors—ranging from deep blues and greens to golds and reds—are selectively reinforced and flashed back to the viewer.
This effect is visible only from specific angles, which is why the color appears to flash and shift as the stone is moved. The vibrant display is a direct consequence of the mineral’s complex structure.
Geological Origins and Mining Locations
Labradorite forms deep within the Earth in specific types of igneous rock, created by the cooling and solidification of magma or lava. It is most commonly found in mafic igneous rocks, such as basalt and gabbro, and is particularly abundant in anorthosite. The formation of the exsolution lamellae requires a very slow and controlled cooling environment. This gradual process allows the calcium and sodium atoms enough time to separate into the finely spaced, alternating layers.
The mineral derives its name from its type locality, the Isle of Paul near Nain in Labrador, Canada, where it was first documented by Moravian missionaries in 1770. Today, significant deposits are mined in various locations around the world.
Global Sources
Notable sources include Madagascar, which produces a gray-to-black labradorite with a strong color flash. Finland is famous for a rare variety called Spectrolite, known for its exceptional, full-spectrum play of color. Deposits are also found in Russia, Norway, and the United States.