Octahedron

grabcad

For this model, I started off by drawing a square. I then extruded the drawing using the extruded boss/base tool. I made sure the height of the extrusion was half of the size of the sides of the square. Then, using the chamfer tool, I clicked the four edges visible when viewing the top plane (make sure the chamfer goes throughout the entire model). Later, I mirrored the entire model using the bottom face as the mirror face/plane (or you could use the top plane). Then you're left with an octahedron!!