High Quality Content by WIKIPEDIA articles! Penroseite is a rare selenide mineral with formula (Ni,Co,Cu)Se2. It has a gray-steel color and black streak with a hardness of 3. It is an isometric mineral, 2/m3. Penroseite was first discovered in 1925 in a Bolivian rhyolite. It was named for Richard Penrose (1863-1931), an economic geologist.