13 WS2812 LEDs powered by an ATTiny85, and housed under the glass and bezel from a cheap clock. Tinting for the half-mirror effect is provided by two layers of antistatic bagging. You can see more pictures of the setup on the page for my Arc Reactor project.
Circle of glass + bezel
Got mine from a small clock.
Two layers of antistatic bag for tinting
Or you could buy several square feet of the real stuff...