Bluefire Type 27 Depressed Center Wheel, 4-1/2 in x 1/8 x 7/8, 30 Grit, Zirconia Alumina A blend of R&D advanced chemistry and shaped zirconia alumina abrasive and premium aluminum oxide grain. Up to 20 percent increase in performance and life over our former zirconia alumina wheels. Excellent performance-to-price ratio