In school I visited the Beineke Rare Books Library at Yale University, a near windowless box whose walls were made of inch-thick slabs of marble that were nonetheless thin enough for sunlight to show through.
I wondered if it was possible to make concrete thin enough to get it to admit light. Even at 1/16" it still won't admit light (and yes, I know about LiTraCon). Instead I chose to cut slots into the concrete. At first one slot on each side, and then several. Because most people think of concrete as an especially heavy, clumsy material, I enjoy when I can execute a design that allows it to show off its elegant side.
The cube is 3" square and a 3" tall plus the base - but it was also done in a single cast. The candlebox below is identical except it has multiple slots per side. In order to give the vertical elements in between the slots more stability, I added cross connections at random heights.
These weren't that hard to make, but I only ever made two of them, I'm not sure why I didn't explore the idea of piercing other patterns into the sides of the cubes. Maybe this is something I'll follow up with in the future.