ConchoidalWalter Schumann, Gemstones of the world (2001), ConchoidalWalter Schumann, Gemstones of the world (2001)
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SiO 2(+Al 2O 3)Walter Schumann , Gemstones of the world (2001)
SiO 2 · nH 2OGemdat.org , Management Team (2012)