Dice: The one in the middle is an Octahedron.
An octahedron is a polyhedron (a 3D shape) with eight sides, which are all equilateral triangles, four of which meet at each corner. It has 12 edges and 6 corners. It is one of the 5 Platonic solids.
Simply put, this is two rectangular pyramids put together. A rectangular pyramid is a pyramid with the base of a square or rectangle. In other words, this is just a pyramid that, when flipped to stand on its point (like a spinning top) it has a rectangular from a birds eye view.
Octahedral shapes also occur in nature: The crystals of Diamond, alum, and fluorite have this shape. Certain dice also have this shape.
Images for kids
The octahedron represents the central intersection of two tetrahedra
Two identically formed rubik's snakes can approximate an octahedron.