Peristerite

Peristerite is a gem variety of sodium-rich plagioclase feldspar (in the albite-oligoclase range) that displays a soft blue or bluish-white iridescence. The name derives from the Greek for pigeon, because its play of color recalls the blue sheen on a pigeon's neck.

It is essentially a plagioclase moonstone, distinct from the more common potassium-feldspar moonstone (adularia). Its shimmer comes from light scattering off extremely fine alternating lamellae of two slightly different plagioclase compositions.

Translucent and pearly, peristerite is cut as cabochons and beads and is valued as an affordable, attractively glowing alternative to classic moonstone.

Peristerite forms when a sodium-rich plagioclase that crystallized as a single phase unmixes during slow cooling into two intergrown compositions - one near albite and one near oligoclase - across the so-called peristerite immiscibility gap (roughly An2 to An17). The nanoscale lamellae produced by this exsolution diffract light and generate the blue sheen.

It occurs in slowly cooled plutonic and metamorphic rocks, including pegmatites, granitic rocks, and gneisses. Notable sources include Ontario and Quebec in Canada, Virginia in the United States, Norway, and parts of Tanzania and Sri Lanka.

Peristerite is white to colorless with a blue or silvery sheen that floats across the stone (adularescence/schiller), hardness 6-6.5, vitreous to pearly luster, and white streak. It shows the two near-90-degree feldspar cleavages and often fine albite-twin striations.

It closely resembles classic adularia moonstone; the practical distinction is chemical (peristerite is a sodium plagioclase, adularia a potassium feldspar) and the often cooler, more silvery-blue tone of peristerite. It differs from labradorite, which shows multicolored flashes (labradorescence) rather than a single floating blue glow.

Peristerite is used as an ornamental and gem material, cut into cabochons, beads, and carvings to display its blue sheen. It serves as an inexpensive alternative to fine Sri Lankan moonstone and is popular in artisan and bohemian-style jewelry.

Value depends on the strength and evenness of the blue sheen and the clarity of the stone. In the crystal and metaphysical trade it is grouped with moonstone and associated with calm, intuition, and emotional balance, though these are spiritual rather than scientific claims.