Certain impurities of zircon sands, especially alumina, titania, calcia and yttria cannot be completely removed in the production of fused zirconia and may have an influence on the corrosion resistance and other product properties of refractories for continuous casting of steel. In this paper, we present our findings on how impurities in raw materials end up in different stabilised zirconia refractory grains, in particular calcia-, magnesia- and yttria-stabilised zirconia. The microstructure (and phase composition) is affected by both the raw materials and the fusion conditions (furnace type and cooling technology).