Hearts, skulls and bone.

Anatomical illustration already speaks the same language as microrealism: hatching, cross-hatching, greys that describe volume without needing a hard outline. A heart is a knot of vessels and those vessels are the entire drawing; a skull is defined by how the light drops into the socket and under the cheekbone, and load it with black and it stops being bone and becomes a smudge. These age well, because the detail lives in the form rather than in the delicacy of the grey.