Plaster, made of calcium sulfate hemihydrate (CaSO4•1⁄2H2O) and/or γ-anhydrite (CaSO4) is obtained by dehydration of gypsum (CaSO4•2H2O) at 100-200 °C. When mixed with water, it dissolves while new gypsum crystals precipitate. Plaster’s microstructures affect its reactivity and setting properties, but are poorly understood due to confusion between crystals and polycrystalline structures in the literature. Through a multi-scale approach combining X-ray microtomography, optical and scanning electron microscopy, we revealed size, morphology and orientation of pores and crystals formed by dehydrating single crystals at T = 105-130 °C and pH2O = 0-40 hPa. The expulsion of water from the (010) interlayers results in a three-level fragmentation: cracks form parallel to (010), then the solid is divided into [101] polycrystalline needles (5-10 x 30-300 μm) formed of micron-sized crystals arranged in a fishbone pattern. The same behavior is observed whatever T and pH2O, however the crystallites size increases with pH2O.