Kashmir repeated twinning, also called polysynthetic twinning, is a crystallographic feature observed in some Kashmir sapphires in which closely spaced parallel twin lamellae form during crystal growth. Twinning is common in corundum across all origins; the relevance of repeated twinning to Kashmir attribution is that, in combination with other features, it forms part of the broader pattern that laboratories examine when establishing origin.

Polysynthetic twinning in corundum produces visible parallel striations under crossed polarisers, with each lamella representing a thin slab of crystal in twin-related orientation to its neighbours. Where the lamellae are very closely spaced and oriented along the basal plane the feature can be conspicuous in transmitted polarised light. The phenomenon occurs in sapphires from many origins and is not by itself origin-diagnostic, but the specific morphology and density of repeated twinning in Kashmir material adds to the overall fingerprint that supports a Kashmir laboratory call.

For the working trade the term is most useful as part of the vocabulary of Kashmir laboratory reasoning rather than as a feature the dealer would identify independently. The relevant inclusions and structural features are documented in the published reference materials of SSEF, Gübelin and AGL.