A zircon halo is a fine internal feature consisting of a small crystal of zircon surrounded by a discoidal or spherical pattern of stress fractures within a host gem. In Madagascan sapphire, particularly stones from Ilakaka, Andranondambo and certain parts of the Ambatondrazaka and Bemainty fields, zircon haloes are a frequent and useful diagnostic inclusion.

The fractures form because zircon contains trace radioactive elements (uranium and thorium) whose decay over geological time damages the surrounding crystal lattice. The volume of the inclusion expands slightly as the zircon's own structure becomes metamict, generating tensile stress in the host that ultimately produces the halo of tension cracks. The pattern is sometimes called a tension halo or a metamictisation halo in the technical literature.

For the gemmologist, zircon haloes are valuable evidence of natural metamorphic origin: they will not form in synthetic corundum and are very rarely encountered in basalt-related stones, which tend to have a different inclusion suite. Their presence is a strong, although not absolutely conclusive, indicator of a metamorphic origin compatible with Madagascar, Sri Lanka, Kashmir or East Africa, and they help separate Madagascan and Sri Lankan natural sapphire from flame-fusion and flux-grown synthetic counterparts. The fractures may also act as conduits for foreign material during heat treatment, so their appearance can shift after standard heating, an additional clue when assessing treatment history.