I made a strange piece of modern furniture with concrete and LEDs. The final product looks like this:
I built this by first constructing a mold out of MDF, and by embedding LEDs into the surface of the mold. The control/batteries are located behind a panel on the bottom, and all the wires are routed through the concrete:
The trickiest part of making the mold was creating a circular groove in which to seat the carboard tube. I used a router trick to do this. First, make a face plate for the router, featuring an offset bolt (the center). Bolt this your workpiece, and then route in a circle:
After all of my work, there are some imperfections. The concrete didn't fully flow into some of the narrower places. I think I could have avoided these problems by using a concrete vibrator, and by choosing concrete without gravel:
The electronics that drive this are very simple. I use a microcontroller to generate a PWM signal for each of the six channels. Each one drives a chain of LEDs through a transistor.
I detail the construction in photos on flickr.