Discovered in 1813 and named after French geologist Louis Cordier, natural transparent crystals are known in gemology as iolite or dichroite for their strong violet-to-yellow pleochroism.
Extruded synthetic cordierite (Mg2Al4Si5O18) aligns its orthorhombic crystals to exploit its ultra-low thermal expansion, preventing cracking in automotive catalytic converter honeycomb substrates.
Its resistance to sudden temperature swings also makes synthetic cordierite the standard ceramic for kiln shelves, pizza stones, and electric heater insulators.