A lily pad is a disk-shaped inclusion most commonly observed in peridot, the gem variety of forsterite-rich olivine. The inclusion consists of a small solid central crystal, often a chromite, surrounded by a flat, roughly circular halo of stress fractures and tiny fluid-filled cavities. Viewed under magnification with a darkfield illuminator, the structure resembles a lily pad floating on water, with the central crystal as the flower bud at the centre. The feature is sufficiently characteristic of natural peridot that gemmologists treat its presence as a strong indicator of natural origin and of olivine identity.

Formation

Lily-pad inclusions form during the slow cooling of the host olivine. The central crystal, typically chromite or another oxide, has a different coefficient of thermal expansion than the surrounding forsterite. As the host crystal contracts during cooling, the mismatch produces radial stress around the included mineral. Fluid trapped in the surrounding lattice migrates into the resulting micro-fractures, creating the characteristic disk-shaped halo of secondary cavities and incipient feathers. The flat orientation of the disk reflects the cleavage and growth structure of the host olivine.

Diagnostic significance

Lily-pad inclusions are diagnostic of peridot in two senses. First, they identify the host as olivine, since the same morphology is rarely observed in any other gem species. Second, they identify the stone as natural, since lab-grown forsterite (produced by flux or hydrothermal methods on a small scale) does not typically reproduce the characteristic disk geometry. For laboratory grading purposes the lily pad is a classic textbook inclusion, illustrated in essentially every gemmological photo-atlas including the GIA reference series.