sillimanite

Sillimanite is a gemstone that has the same chemical composition as kyanite and andalusite, but has a different crystal shape. Those produced in the form of fibers are sometimes called "fibrolite." It comes in colors such as colorless, blue, brown, yellow, and green, and it is also characterized by its strong pleochroism, meaning that the colors appear different depending on the viewing angle. Some exhibit a cat's eye effect, where when exposed to light, a ray of light appears like a cat's eye.

Karatz GemMagazine
" Sillimanite "