The Schwartz D surface is a minimal surface discovered by Karl Hermann Amandus Schwarz in the 1880s (the D stands for "Diamond"). This surface has no boundaries and it has been cut here with a rhombic dodecahedron (the structure of a garnet crystal) which gives a much nicer object than the simple cubic cut shown on Wikipedia
. This version is the true minimized surface, produced using Surface Evolver (and not the more usual pseudo surface obtained by trigonometric equations).
The surface has been densely perforated with hexagons by a small Python script to allow a more complete visualization of its inside and to optimize the printing cost.