A spark plug includes an insulator formed of a ceramic material. The ceramic material comprises Al2O3 in an amount of 98.00 wt % to 99.50 wt %; Group 2 oxides in an amount of 0.16 wt % to 0.70 wt %; SiO2 in an amount of 0.25 wt % to 0.75 wt %, Group 4 oxides in an amount of 0.01 wt % to 0.16 wt %, Group 1 oxides in an amount less than 0.0060 wt %, and P2O5 in an amount of less than 0.0040 wt %. The Al2O3 is formed of particles having a D50 median particle size by volume of 1.2 μm to 1.8 μm. The ceramic material is pressed, sintered, and formed to a predetermined shape. The sintered ceramic material includes a glass phase comprising the Al2O3, Group 2 oxides, and SiO2. The sintered ceramic material also includes secondary crystals of calcium hexa-aluminate (CaAl12O19) spinel (MgAl2O4), anorthite (CaAl2Si2O8), and mullite (Al6Si2O13).