Dumortierite is a rhombic mineral, boron silicate of aluminum and iron. It was described as early as the 12th century as an alleged lapis lazuli at a site in Beaunan near Lyon (France), where it occurs in garnet rule along with cordierite and sillimanite. Dumortierite usually forms thinly columnar, acicular to fibrous agglomerates radially or parallel. The scratch is white. Dumortierite is clearly fissile, translucent, with a glassy to silky sheen. Hardness 7 Mohs scale.