Lamellar twinning describes the repeated, parallel intergrowth of crystal twins on a closely spaced set of planes, producing a stack of thin twin lamellae within a single grain. In gemmology the feature is most familiar in corundum, where twin planes parallel to the rhombohedron generate the stepped, fingerprint-like striations that gemmologists detect with darkfield illumination and immersion. It is also routinely seen in plagioclase feldspar (the basis of the petrographer's albite-law and pericline-law twinning) and in calcite, where it underlies the well-known mechanical twinning observed in deformed marbles.

Optical signature

Under the microscope, lamellar twinning shows as a set of parallel reflective planes, sometimes called polysynthetic twinning, that catch light as the stone is rotated. In corundum the planes commonly occur on two or three intersecting orientations and may be associated with intersecting boehmite needles where fluid has migrated along the twin junctions. In plagioclase the lamellae produce the alternating extinction stripes diagnostic in petrographic thin section. Lotus Gemology and GIA training literature treat dense lamellar twinning in ruby and sapphire as a strong indicator of natural origin, since synthetic flame-fusion corundum rarely develops the same stacked geometry.

Origin

Twin lamellae form by two mechanisms: growth twinning, where the lattice nucleates in twin orientation during crystallisation, and deformation twinning, where lattice planes shear into a twin position under directed stress. Corundum lamellae are predominantly deformation features acquired during regional metamorphism. Calcite e-twins in marble are the textbook case of mechanical twinning and are used by structural geologists as a strain gauge. Plagioclase combines both mechanisms and the orientation of the lamellae carries information about the rock's thermal history.

Bearing on durability and treatment

Tightly spaced twin planes can act as planes of weakness. Cutters working ruby and sapphire orient the table to avoid having a major twin plane parallel to the girdle, since impact on the girdle can propagate a fracture along the lamellae. Heat treatment of corundum sometimes causes precipitates along the twin planes to dissolve, leaving the lamellae visually less prominent but the planes themselves intact. The presence of dense lamellar twinning is therefore a clue both to natural origin and to the orientation a cutter chose to work around.