Conchoidal, ConchoidalArthur Thomas, Gemstones (2009)
Brittle, UnevenArthur Thomas, Gemstones (2009), Brittle, Uneven
(Ca,Fe,Mn,Mg) 3Al 2BSi 4O 15(OH)Walter Schumann , Gemstones of the world (2001) More from other references
(Mg,Fe)Al 3(BO 4)(SiO 4)OUlrich Henn and Claudio C.