If you want to make some sort of electronic jewelry, there are many options. Something small like an Arduino Nano or Adafruit Trinket would generally fit the bill, but then you need to find somewhere to fit sensors, LEDs, a power source, and whatever other interesting components you decide to add.
For a simple solution, to this challenge, Relabs has come up with the LightMini. The 30 x 30 mm board features an ATtiny85 processor for control, and has a built-in connector for a coin cell battery. It comes with either 4 or 6 programmable LEDs for display purposes, and can be outfitted with a photoresistor or thermoresistor, allowing the device to react to its environment.
The device can be ordered with firmware already loaded, but you can program it as an ATtiny85, using an Arduino as an ISP (in-system programmer). Information on this technique can be found in this high-low tech article, which is great information for programming an ATtiny in any situation. If you only need a limited number of I/O, at a very low price, having a few “tiny” microcontrollers in your toolbox is a great option!