Irregular/uneven, Uneven
ConchoidalArthur Thomas, Gemstones (2009), ConchoidalArthur Thomas, Gemstones (2009), Brittle, Uneven
Ca 2Al 3(SiO 4) 3(OH)Walter Schumann , Gemstones of the world (2001) More from other references
Mg 3Al 6(Si,Al,B) 5O 21(OH)Walter Schumann , Gemstones of the world (2001) More from other references