Inc. Diamond is the hardest mineral, number 10 in the Mohs scale of mineral hardness . The flat triangles on the diamond correspond to the faces of the octahedron. Diamond crystals usually have rounded, grooved edges as can be seen next to the flat faces. The images show diamond's adamantine luster , the distinctive brilliant greasy surface appearance that, along with its extreme hardness, distinguishes diamond from all other minerals. Only quartz resembles it, but that mineral is softer and has a glassy luster.
Diamonds form at great depth, well below the crust.
They reach the surface only in peculiar eruptions that explode into the air, leaving behind bottomless pipes of mantle rocks called kimberlites. See this article for more about their geology. I think this stone may be a twinned crystal. The two pictures show opposite sides, and the knob at the lower right of the left image is on the upper left of the right image. See if you can picture its three-dimensional shape.