This is a reauleaux tetrahedron, meaning that from any point on the surface of the shape to the opposite side of the shape (passing through the centre) takes the same distance. As a result, the tetrahedron can "roll" smoothly between two flat surfaces, as they will remain a constant distance from one another. tl;dr It's a cool shape!
90% or solid for rolling performance
This .stl file may be off by around 0.01 mm, and printing may introduce yet more deviations from a perfectly symmetrical shape. Be prepared to sand the print slightly to improve the symmetry of the tetrahedron, and allow for this by having a decent outer perimeter width.
The shape is formed by intersecting four evenly sized spheres, and only keeping the overlapping part of the shape. I did this is 123d design, but I don't know how easily done this operation is in other 3DCAD or modelling softwares (but hey, 123d design is free so if you can't do this in your program just get it)!