Chunky Rough Kyanite Crystal

The colour of this medium grade kyanite crystal becomes more intense when held up towards the light.

Some kyanite crystals are more translucent than others.

One interesting property of kyanite is that its hardness varies depending on the direction in which it's scratched. This characteristic is known as anisotropic. 

Being anisotropic can affect the way a material behaves in different situations.

Anisotropic materials have different physical properties in different directions.  Isotropic materials have the same physical properties in all directions.

When a kyanite crystal is tested lengthways it grades between 4.5 to 6 on Mohs scale of hardness.  When tested across the width of the same crystal it's 6 to 7.

Kyanite is widely used in industry not only because of its hardness but also because of its ability to resist heat.