The microhardness H_V of gadolinium calcium oxyborate single crystals has been investigated on the planes of different orientations as a function of applied load and indenter orientation. It was found that: (1) on the (010) plane microhardness is practically independent of indenter orientation and indenter diagonal d, and is constant, (2) on the (100) and (001) planes both normal and reverse indentation size effects occur in the indentation diagonal intervals below and above a critical value d_c, and (3) at loads exceeding about 30 g all indentation impressions are accompanied by cracks, the formation of which does not depend on the orientations of indentations. The observations of indentation size effect were analysed by using Hays-Kendall's approach, the proportional specimen resistance model of Li and Bradt, and the strain gradient plasticity theory. The analysis of the data revealed that: (1) indentation size effect on the different faces of gadmium calcium oxyborate may be explained satisfactorily by the proportional specimen resistance model and the strain gradient plasticity theory, (2) the models explain the dependence of microhardness H_V on indentation diagonal d in the range d < d_c, while for deformation at d > d_c the excess indentation pressure, and the energy associated with it, is expended in creating and developing radial and lateral cracks around indentations, and (3) in the range d < d_c the load-independent microhardness H₀ on different planes changes in the sequence: H₀(010) > H₀(001) > H₀(100), and may be explained in terms of the process of rupturing of successive planes perpendicular to the direction of penetration of indenter.

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