Sew a little sparkle into your wearable project with an Adafruit LED Sequin. These are the kid-sister to the popular Flora NeoPixel, they only show a single color and they don't have digital control, but that makes them smaller easier to use for many projects.
Simply connect 3 to 6VDC to the + pin and ground to the - pin, and the LED on the board will light up. You can make the LEDs fade and twinkle by using the PWM (a.k.a. analogWrite) functionality of your Gemma or Flora, or just connect directly to a digital I/O pin of a microcontroller to turn on and off. Or even skip the micro altogether, and power directly from a LiPoly or coin battery.
These LEDs are 5 Ruby Red "1206 size" LEDs, matched with a 220 ohm resistor. When powered from 3.3V they draw about 5mA so you can put up to 4 or 5 in parallel on a single microcontroller pin.