We report on the examination of the statoliths of the coronate medusa Periphylla periphylla. The statoliths are single crystals of calcium sulfate hemihydrate of about 60 μm length and 15 μm diameter, as identified by powder and single-crystal diffractometry at the synchrotron. This is most surprising, as this phase (gypsum plaster, the mineral bassanite) can not usually be precipitated from aqueous solutions and easily takes up water to yield the dihydrate of calcium sulfate (hardening reaction of gypsum). As it is sensible to use the considerably more dense hemihydrate instead of the dihydrate as a gravity sensor (32% higher density after buoyancy correction), it can be concluded that this biological system actively induces the crystallisation of this phase far away from chemical equilibrium. As a biomineral, calcium sulfate hemihydrate has not been found in living organisms before. The results are complemented by a discussion of histology and evolutionary aspects.