We inscribed thick volume gratings in WMS-15 glass ceramic by ultraviolet light at 193 and $248\mathrm{nm}$ . Unlike earlier work in ceramic materials, the inscription process modified the optical properties of the material without the need for any additional chemical or thermal processing. Experimental evidence from measurements of grating growth, thermal annealing, and spectral absorption indicates that two distinct physical mechanisms are responsible for the grating formation. Weak, easily thermally bleached gratings resulted from exposure fluences below $0.3\mathrm{kJ}/{\mathrm{cm}}^2$ . Optical absorption measurements suggest that these low fluence gratings are predominantly absorption gratings. More thermally stable gratings, found to be refractive index gratings with unsaturated refractive index modulation amplitude as large as $6\times {10}^{-5}$ were formed at cumulative fluences of $1\mathrm{kJ}/{\mathrm{cm}}^2$ and above.