Agate

6.5 to 7Herve Nicolas Lazzarelli, Blue Chart Gem Identification (2010) 2.60 to 2.64Walter Schumann, Gemstones of the world (2001) More from other references NoneWalter Schumann, Gemstones of the world (2001) UnevenWalter Schumann, Gemstones of the world (2001) 1.530 to 1.543Herve Nicolas Lazzarelli, Blue Chart Gem Identification (2010) Uniaxial/+Ulrich Henn and Claudio C. Milisenda, Gemmological Tables (2004) More from other references up to to 0.004Walter Schumann, Gemstones of the world (2001) More from other references AbsentWalter Schumann, Gemstones of the world (2001) NoneWalter Schumann, Gemstones of the world (2001) Banding of different colours. Colours are generally pale though varied; natural colours are green, yellow, red, reddish brown, white and bluish white, among others.Michael O?Donoghue, Gems, Sixth Edition (2006) Translucent,OpaqueUlrich Henn and Claudio C. Milisenda, Gemmological Tables (2004) More from other references Waxygemdat.org, Management Team (2012) Varies with bands: partly strong; yellow, blue-whiteWalter Schumann, Gemstones of the world (2001) Trigonalgemdat.org, Management Team (2012) Microcrystalline aggregatesWalter Schumann, Gemstones of the world (2001) More from other references It occurs in filling cavities, and individual bands are concentric to the external surface of the mass or nodule. Agate geodes are found in basic lavas and other igneous flow rocks and have probably been formed by silica deposition in cavities created by gases.Michael O?Donoghue, Gems, Sixth Edition (2006) Agate information at mindat.org Brazil  Peru  UK  [Specimen in the Natural History Museum, London] USA  U.S. Geological Survey, 2005, Mineral Resources Data System: U.S. Geological Survey, Reston, Virginia. U.S. Geological Survey, 2005, Mineral Resources Data System: U.S. Geological Survey, Reston, Virginia.