Blue anatase crystals in a quartz from Brazil  

  Small crystals of anatase in a colourless Brazilian quartz bead necklace (we can see the drill hole of the bead in the background).   Anatase is one of the three mineral forms of titanium dioxide (TiO2), along with brookite and rutile.   Spectral colours, due to diffraction, occur at the anatase/quartz interface. They are due to the significant difference of refractive indices between anatase (2.48-2.56) and quartz (1.54-1.55).   The observable transparency and bulk colourlessness of these anatase crystals are due to their small size. In general, larger anatase crystals exhibit various colours to a black colour and they frequently appear opaque. The coloured and transparent anatases are cut into collectors' gems (low hardness, from 5.5 to 6).    

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