Kyanite
Source: This is an interesting mineral in that it occurs in metamorphic rocks in the form of the blades that you see above. These can form to be several centimeters long. The longest shown in this photograph is more than 10cm in length. The presence of these kyanite blades can tell geologists much about the pressure and temperature of the formation of the metamorphic rocks (see Colored Gemstones 101) in which it formed.
Chemical: Al2SiO5 an aluminum silicate
Crystal System: Triclinic
Formation: In long blades
Properties: Hardness of 6-7 but quite brittle and will break easily down the long direction of the crystal.
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