KYANITE
The aluminum silicate mineral kyanite (Al2SiO5)
is easy to identify. It has long, bladed crystals, a typically pale blue
to dark blue coloration, and one cleavage plane. Kyanite is the best
example of a mineral with variable hardness - it has a hardness of about 5
along the length of the blades and a hardness of about 7 across
the blades. This mineral is usually encountered in some intermediate- to
high-grade metamorphic rocks.
Kyanite
(upper blade is 4.5 cm long; middle blade is 6.8 cm long; lower blade is 5.0 cm
long)