Fully dense transparent zinc aluminate ceramics with nanoscaled grain sizes were fabricated by Deformable Punch Spark Plasma Sintering (DP-SPS). Optical transmission spectra showed high transparency, with up to 70% transmitted light in the visible spectrum. Vickers hardness was measured and grain boundary strengthening observed, showing hardness increase from 18.2 GPa up to 22.5 GPa as the grain sizes decreased from 60.3 to 10.1 nm. The trend followed the Hall-Petch relationship, with hardness linearly proportional to the inverse of square root of grain size. A low grain size limit reported in previous literature below which hardness decreases, known as inverse Hall-Petch relationship, was not observed within the studied grain size range. Cross-sections of the hardness tests' indentations were prepared by focused ion beam and observed by electron microscopy and showed radically different crack patterns underneath the indentation imprint when contrasting samples with dissimilar grain sizes, shedding light on the mechanisms behind the observed grain boundary hardening mechanisms.