Labradorite is a calcium-enriched feldspar mineral, part of the plagioclase series, recognized for its unusual visual characteristic. This stone often appears as a simple gray or dark gray mineral until light strikes it at a particular angle. Hardness measures a mineral’s resistance to scratching, which determines its suitability for use in jewelry or architectural materials. Understanding this durability is important for its proper handling and application.
The Specific Hardness of Labradorite
Labradorite is rated at 6 to 6.5 on the Mohs Hardness Scale, placing it in the range of intermediate hardness for minerals. This scale, which runs from 1 (softest, like talc) to 10 (hardest, like diamond), is based on the ability of one mineral to visibly scratch another. A rating of 6 to 6.5 means labradorite is harder than steel, which typically rates around 5.5, but it is softer than quartz, which is a common component of dust and rates at 7.
This intermediate rating indicates that while the stone possesses a fair degree of resilience, it remains susceptible to scratching and abrasion from many everyday materials. Common household glass has a hardness of about 5.5, meaning it cannot scratch labradorite. However, a piece of common sand or grit containing quartz can damage its surface. The hardness measurement demonstrates that the stone can withstand general handling but requires consideration to avoid contact with harder substances.
How Hardness Influences Practical Use
The 6 to 6.5 hardness rating has direct implications for labradorite’s use, particularly in jewelry and decorative architecture. For jewelry, the stone is well-suited for items that experience lower impact, such as pendants, brooches, and earrings. However, for rings and bracelets, which are prone to frequent bumps and scrapes, labradorite is considered less durable. It may require protective settings, such as a bezel, to shield the surface from abrasion.
The stone’s durability is also affected by its internal structure, specifically its cleavage. Labradorite possesses two distinct directions of perfect cleavage, meaning it has planes within its crystal structure where the mineral is prone to splitting when subjected to impact. These cleavage planes intersect at angles near 90 degrees, making the stone vulnerable to fracturing from a sharp blow.
In decorative applications, such as countertops, tiles, and facing stone, the moderate hardness and presence of cleavage dictate careful placement and maintenance. While the stone can be cut and polished for architectural use, it is generally best avoided in high-traffic or high-wear areas where it might be exposed to abrasive cleaning or heavy impact. Lapidaries must also consider the cleavage when cutting and polishing the stone, as incorrect orientation can result in a fracture.
Labradorescence and Other Identifying Characteristics
Labradorite’s most distinguishing feature is its optical effect, known as labradorescence. This phenomenon is the brilliant, iridescent flash of color that appears when light reflects off specific internal structures within the stone. The shimmering colors, which often include electric blue, green, gold, and sometimes red, are caused by the interference and diffraction of light as it interacts with ultra-thin, parallel layers inside the crystal.
The structural cause of this play-of-color is a result of the mineral’s composition as a plagioclase feldspar, which is a solid solution of calcium and sodium aluminosilicates. The layers that create the color flash are a product of slow cooling, which allows the components to separate into fine, alternating lamellae. Other physical characteristics used for identification include its triclinic crystal system and a specific gravity typically ranging from 2.68 to 2.72.