Peristerite is a variety of plagioclase feldspar in the albite–oligoclase compositional range that exhibits a soft, often diffuse blue or white adularescence. The name derives from the Greek peristera, meaning pigeon, an allusion to the iridescent sheen on a pigeon's neck feathers. Peristerite occupies a niche position in the gem trade: visually adjacent to moonstone but structurally distinct, more often a collector's curiosity than a mainstream commercial material.

Composition and structure

Peristerite forms within the plagioclase solid-solution series, with compositions typically between An₂ and An₁₇ — the albite to sodic-oligoclase range. Below the solvus, the originally homogeneous plagioclase unmixes into alternating lamellae of more sodic and more calcic phases. Where the lamellar spacing approaches the wavelength of visible light, scattering at the phase boundaries produces the characteristic peristerite sheen. The effect is usually softer and less directional than the adularescence of fine orthoclase moonstone, and the colour tends toward a milky blue-white rather than the electric blue of the finest Sri Lankan moonstone.

The species is triclinic, with a hardness of 6 to 6.5, specific gravity around 2.62 to 2.65, and refractive indices of approximately 1.53 to 1.54. Cleavage is good in two directions and must be respected in cutting.

Sources

The classic source is the Bancroft district of Ontario, Canada, where peristerite occurs in pegmatites and metamorphic rocks of the Grenville Province. Bancroft material has been a staple of Canadian gem-show offerings for decades. Lesser quantities have been reported from Scandinavia, particularly from pegmatites in southern Norway, and from scattered occurrences in the United States and India. None of these sources approaches the commercial scale of Sri Lankan or Indian orthoclase moonstone.

In the trade

Peristerite is cut almost exclusively en cabochon, oriented to display the sheen across the curved dome. Faceted material is occasionally seen but rarely impresses, as the body lacks the brilliance of a higher-RI gem. Buyers should expect modest prices reflective of the limited demand and the diffuse character of the sheen. Material is sometimes mislabelled simply as moonstone in mixed-parcel offerings, and the distinction matters mineralogically even where it does not greatly affect price. Laboratory identification by refractive index or composition is straightforward when called for.

Identification

The triclinic crystal system, the plagioclase composition, and the higher refractive index distinguish peristerite from monoclinic orthoclase moonstone. Polysynthetic albite twinning is often visible under magnification on cleavage surfaces, an immediate flag for plagioclase rather than orthoclase. The sheen itself, when soft and diffuse rather than sharply directional, is consistent with peristerite identification but is not diagnostic on its own.

Care

Hardness at 6 to 6.5 makes peristerite a soft stone for daily-wear ring use. Bezel settings in pendants and earrings are the appropriate format. Cleansing should be by mild soap and warm water; ultrasonic and steam cleaning are not recommended because of the cleavage.

Further reading

• GIA — Feldspar reference
• International Gem Society — plagioclase varieties
• GIA Gems & Gemology — feldspar articles