Conchoidal, ConchoidalArthur Thomas, Gemstones (2009)
Conchoidal, ConchoidalAnthony et al, Handbook of mineralogy (2001), Brittle
(Ca,Fe,Mn,Mg) 3Al 2BSi 4O 15(OH)Walter Schumann , Gemstones of the world (2001) More from other references
SiCArthur Thomas , Gemstones (2009)