adularia

Adularia, also known as adularite, is a colorless to pale white variety of the feldspar mineral group, typically classified as a potassium feldspar (KAlSi3O8). It occurs in igneous and metamorphic rocks, particularly in hydrothermal veins and pegmatites. The name derives from Adular, a locality in the Swiss Alps where the mineral was first described, and the term is closely associated with the moonstone gem.

In gemology, adularia is valued for adularescence, a soft bluish sheen that seems to float beneath the surface. The effect arises from microscopic lamellae within the feldspar, often a result of exsolution or intergrowth of different feldspar components, which scatters light to produce the characteristic glow. Colors range from colorless to milky gray, with the best stones displaying a bright blue sheen when cut en cabochon to maximize the adularescent effect.

Historically, adularia has been linked with moonstone, a feldspar gemstone where adularescence is prominent. In modern mineralogy, moonstone is regarded as a gem-quality feldspar and adularia describes the classic moonstone variety derived from orthoclase/microcline assemblages. The term persists in gem trade and older literature as a traditional name for these moonstone-form feldspars.

Physical properties commonly cited include a Mohs hardness around 6 and typical cleavage in two directions at or near 90 degrees, contributing to the need for appropriate care in jewelry. Adularia remains the traditional reference point for the moonstone-like, adularescent feldspars used in decorative stones.