Step 1: The Circuit
I used three auto-change RGB leds. These leds have a tiny chip built into them that causes the colors to shift in random patterns. Because of this though, they can not be wired in series, only in parallel. Since I wanted to power this device with a 9 volt battery and each led is only rated for 3 volts, I also needed to add resistors to protect the leds. I used three 100 ohm resistors wired in series with the leds, which seem to work well in this circuit. Lastly, it has a simple toggle switch for turning it off and on.
Step 2: Building the Base
This is the base I built to hold the components (before staining). I made it completely out of scrap wood. Specifically, a piece of 2x4 leftover from building my workbench, a small scrap of plywood, and some old supports that at one point used to hold up my bed.
I carved a hole in the 2x4 for the piece of agate to rest in by first drilling a series of holes and then using a chisel to connect the holes. I finished by filing the hole bigger until the agate fit perfectly.
Step 3: Finished
Here is the finished product. I wired the circuit using some heavy gauge copper wire (left over from wiring my workshop) which I left exposed. This isn't necessary, but I liked the way it looked. The resistors are recessed into the led holes, and I used some heat-shrink tubing to separate the anode and cathode led wires from short circuiting. The metal toggle switch was salvaged from a dead power supply. The piece of metal securing the front of the agate is a modified soldering iron rest. I also sanded the leds so the light would be more diffused across the agate.
All in all, I'm very happy with the results. It creates a hypnotising display of colors in the corner of our living room, which seems to make the wife happy (she loves this sort of thing) and it is especially fun to watch at night.