The non-metallic mineral among the following is :

Chromite is an ore of chromium, bauxite is an ore of aluminium and tungston are metallic in nature while graphite is an allotrope of carbon and a carbonaceous compound. It is a very soft allotrope of carbon while other allotrope of carbon diamond is the hardest known element on this earth. This is because of the chemical structure of graphite which shows slippery nature of layers of carbon and presence of loose electrons.