Albite is the sodium-rich end-member of the plagioclase feldspar solid-solution series, with the idealised chemical formula NaAlSi₃O₈. It forms one pole of the continuous compositional range that extends through oligoclase, andesine, labradorite, bytownite, and anorthite toward the calcium-rich end-member. As a gemstone, albite occupies a narrow but genuinely interesting niche: transparent facetable material is rare and seldom seen in commerce, while translucent to semi-transparent specimens exhibiting a blue to bluish-white adularescence — a phenomenon in plagioclase feldspars termed peristerescence or, more commonly, peristerism — are occasionally cut and sold under the trade name peristerite. The distinction between albite-based peristerite and the orthoclase-based moonstone that dominates the gem market is commercially important yet frequently blurred in retail descriptions.

Crystal System and Physical Properties

Albite crystallises in the triclinic system, typically forming tabular to platy crystals with two prominent cleavage directions intersecting at approximately 86° and 94° — a defining characteristic of the plagioclase group. The hardness on the Mohs scale is 6 to 6.5, placing it in the same range as other feldspars and making finished gems susceptible to abrasion in everyday wear. The specific gravity ranges from approximately 2.60 to 2.65, and the refractive indices fall between roughly 1.527 and 1.538, with a birefringence of approximately 0.011. The optical character is biaxial negative. The lustre on fresh surfaces is vitreous, and the streak is white.

In its pure form, albite is colourless to white. Natural impurities or structural defects can introduce pale grey, greenish, or yellowish tints, though strongly saturated colours are not characteristic of the species. Transparency ranges from fully transparent in the finest pegmatite crystals to translucent or even opaque in massive or twinned material.

Twinning and the Origin of Peristerism

Albite is notable for its characteristic polysynthetic twinning — repeated, fine-scale alternation of twin lamellae on the albite twin law — which produces the fine striated surfaces visible on cleavage faces and is a key diagnostic feature in hand-specimen identification. In gem-quality peristerite material, the optical phenomenon arises from a different but related structural feature: sub-microscopic exsolution lamellae of two compositionally distinct plagioclase phases, one close to pure albite and one closer to oligoclase in composition, that have unmixed during slow cooling. The thickness and spacing of these lamellae, typically on the order of visible-light wavelengths, cause constructive interference and scattering of light, producing the characteristic floating blue to white sheen that resembles — but is physically distinct from — the adularescence of orthoclase moonstone. The quality and colour of the sheen depend on lamellar spacing: finer lamellae tend to produce a bluer, more intense effect.

Occurrence and Notable Localities

Albite is one of the most abundant minerals in the Earth's crust, occurring widely in granites, pegmatites, metamorphic schists, and hydrothermal veins. Gem-relevant material, however, is far more restricted in distribution.

• Canada (Ontario and Quebec): Peristerite of gem quality has been recovered from localities in the Canadian Shield, particularly in Ontario, where coarse pegmatites have yielded translucent material with attractive blue sheen. Canadian peristerite has a modest but established place in the lapidary trade.
• United States: Localities in New England, including parts of Connecticut and Maine, have produced albite crystals of collector interest, though facetable transparent material is uncommon.
• Brazil: Brazilian pegmatites, renowned for producing a wide range of feldspar gem materials, yield albite both as transparent crystals and as the matrix mineral in which other gem species are embedded.
• Norway: Historic localities in the Oslo region have produced albite of mineralogical significance.
• Myanmar and Sri Lanka: Plagioclase feldspars with sheen effects are recovered from alluvial and metamorphic deposits in both countries, though distinguishing albite-dominant peristerite from orthoclase moonstone in the field requires gemmological testing.

Gem Use and Trade Nomenclature

The commercial use of albite as a gemstone is complicated by overlapping and imprecise trade terminology. The name moonstone, as defined by the GIA and the ICA, properly refers to orthoclase feldspar (KAlSi₃O₈) displaying adularescence, a species distinct from the plagioclase feldspars. However, peristerite — albite or albite-oligoclase intergrowths showing a comparable blue sheen — is frequently sold under the moonstone label in both wholesale and retail markets, particularly in South and Southeast Asia. Gemmologically rigorous laboratories distinguish the two by refractive index, specific gravity, and spectroscopic characteristics, but the separation is not always made in the trade.

Transparent, facetable albite is occasionally cut for collectors and specialist dealers. Such stones are typically colourless or faintly tinted, with modest brilliance owing to the relatively low refractive index. They hold limited commercial appeal beyond the collector market but are of genuine gemmological interest as examples of a species rarely encountered in faceted form.

Albite also appears as a significant component in several composite or intergrowth gem materials. Perthite and antiperthite — intergrowths of alkali feldspar and plagioclase — may contain substantial albite. The ornamental stone amazonite (a green microcline feldspar) is sometimes found intergrown with albite, and albite forms part of the matrix in many gem-bearing pegmatites without itself being the gem of interest.

Identification and Separation from Orthoclase Moonstone

Separating albite-based peristerite from orthoclase moonstone is a routine gemmological task with practical commercial implications. Key distinguishing properties include:

• Refractive index: Albite yields readings of approximately 1.527–1.538, while orthoclase moonstone reads approximately 1.518–1.526. The separation is measurable on a standard refractometer, though the overlap between species and the difficulty of obtaining accurate readings on cabochon material can complicate matters.
• Specific gravity: Albite (approximately 2.62) is marginally denser than orthoclase (approximately 2.56–2.59), though the difference is small enough that hydrostatic weighing is preferable to estimation.
• Inclusions and internal features: Polysynthetic twinning striations, visible under magnification on cleavage surfaces or in transmitted light, are characteristic of plagioclase feldspars including albite, and are absent in orthoclase.
• Sheen colour: While both species can show blue sheen, peristerite more commonly produces a steely or silvery-blue effect; orthoclase moonstone more often shows a warmer, more diffuse blue-white glow. This is a guide rather than a rule.

Treatments and Stability

Albite is not known to be routinely treated for gem purposes. Its relatively modest optical properties and low commercial value provide little economic incentive for enhancement. The stone's two good cleavage directions and hardness of 6 to 6.5 mean that it requires careful handling during cutting and setting, and is not well suited to rings or other high-wear applications. Ultrasonic cleaning is inadvisable; gentle cleaning with warm water and a soft brush is appropriate.

Collector and Mineralogical Significance

Beyond its lapidary applications, albite is of considerable importance to mineral collectors. Well-formed, glassy crystals — sometimes referred to in the mineral trade as cleavelandite when occurring in curved, platy aggregates — are prized display specimens. Albite crystals associated with tourmaline, beryl, topaz, or other pegmatite gem minerals form some of the most visually striking mineral specimens in major collections. The species is also of fundamental importance to petrology and geochemistry as a rock-forming mineral, and its study has contributed substantially to the understanding of igneous and metamorphic processes.

Further Reading

• GIA — Moonstone and Feldspar Group Overview
• International Gem Society — Feldspar Gem Information