What mineral is known as the brown diamond?

Answer 1

All diamonds, regardless of the colour, are formed from carbon. Brown is the most common colour of gem-quality diamond found.

Brown coal -- lignite -- is sometimes called the 'brown diamond'.

Answer 2

The mineral known as the brown diamond is typically referred to as "chocolate diamond." It gets its unique color from the presence of nitrogen impurities during its formation process. Brown diamonds are valued for their distinct color and are often used in jewelry.