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Rubies (Cr: Al2O3 single crystals) with 0.025–0.4 mol% Cr2O3 were prepared by the optical floating zone method, and characterized by X-ray diffraction (XRD), electron paramagnetic resonance (EPR) spectroscopy, optical transmission and photoluminescence (PL). Crystals were free of inclusions and cracks, and had high transmittance in the visible region. In addition to the well-known signals from Cr3+ in the α-Al2O3 structure, the EPR spectra indicated the presence of very weak signals, which may correspond to paramagnetic centres associated with an oxygen vacancy, and an electron-loss oxygen centre, O−. The concentration of these paramagnetic centres was at a minimum for Cr2O3 concentration of 0.15 mol%, and additional Cr3+-derived peaks were also observed with Cr2O3 concentrations >0.15 mol%. The PL spectra also varied with Cr2O3 concentrations; the emission intensities of the PL spectra increased with Cr2O3 concentration for concentrations ≤0.15 mol%, then decreased with the further increases in Cr2O3. Thus the maximum luminescence emission intensity and the minimum concentration of paramagnetic structural defects were observed with 0.15 mol % Cr2O3, and the presence of additional interactions involving Cr3+ ions may contribute to the partial quenching of the luminescence at higher Cr3+ concentrations. Overall, the optical floating zone method represents an effective and economic approach to the preparation of high quality rubies, and their optimum optical properties were observed with Cr2O3 concentrations of 0.15 mol%.