Polysynthetic twinning is the repeated parallel-lamellar twin configuration that arises when a crystal repeatedly switches lattice orientation across a defined twin plane during growth or deformation. In gemmology, the term is used most often in two contexts: the twinning of corundum, where polysynthetic twins are part of the inclusion suite used in origin determination, and the twinning of plagioclase feldspars, where polysynthetic twinning is a primary identification criterion for the species.

In corundum

Polysynthetic twins in corundum form on the basal plane (0001) and on rhombohedral planes, with the basal-plane twins being the most commonly observed in faceted ruby and sapphire. Under magnification the twins appear as fine parallel lines or bands, often associated with networks of healing fractures or with arrays of mineral inclusions aligned along the twin planes. The presence of polysynthetic twinning, particularly in combination with characteristic mineral and fluid inclusions, is one of the suite of features that gemmologists use to attribute origin to specific deposits.

The twins are more commonly observed in corundum from metamorphic terrains — the marbles of Mogok and Luc Yen, the gem gravels of Sri Lanka, and equivalent settings — than in corundum from basaltic magmatic sources such as Australia and Thailand. The reason is that metamorphic corundum has typically experienced post-crystallisation deformation that promotes the development of polysynthetic twinning, while magmatic corundum has cooled relatively quickly without comparable deformation history.

In feldspars

In plagioclase feldspar, polysynthetic twinning on the albite law is the routine and diagnostic feature that distinguishes plagioclase from alkali feldspar. The twinning produces fine striations on cleavage surfaces visible under low magnification and characteristic alternating extinction under crossed polarisers. The twinning is ubiquitous in plagioclase to the point that its absence would itself be remarkable; in alkali feldspar (orthoclase, microcline, sanidine), different twinning laws — Carlsbad, Manebach, Baveno — predominate, and polysynthetic albite twinning is correspondingly rare or absent.

The presence of polysynthetic twinning in plagioclase is also responsible for several of the optical phenomena that distinguish gem feldspars: adularescence in moonstone, labradorescence in labradorite, and play of colour in selected sunstone are all manifestations of light interaction with the lamellar structure produced by the twinning combined with associated exsolution lamellae.

Detection

Polysynthetic twinning is detected by polarised-light examination and by microscopic observation. In faceted gems, the polariscope reveals the banded extinction pattern characteristic of stacked twin lamellae; under microscopy, the twins appear as fine parallel lines or as narrow alternating colour bands when the stone is illuminated obliquely. In references such as the Gübelin Photoatlas of Inclusions in Gemstones, polysynthetic twins are catalogued among the inclusion features used in species and origin identification.

In the trade

For the trade buyer, polysynthetic twinning is most often encountered as a noted feature in laboratory reports on fine corundum, where its presence contributes to origin attribution and to the broader characterisation of the stone. In the broader plagioclase feldspar trade — moonstone, labradorite, sunstone — the twinning is part of the structural fabric of the species and is rarely flagged explicitly, since its absence would call the identification itself into question.

Further reading

• GIA Gems & Gemology — corundum origin and twinning articles
• GIA — Feldspar reference
• International Gem Society — twinning and identification