Arduino GEMMA - Miniature Wearable Electronic Platform is a tiny wearable platform with a lot of might which is a combination of work from both Adafruit and Arduino. The board is based on Attiny85 and it can be programmed by Arduino IDE over USB. The purpose of Adafruit and Arduino while creating Arduino GEMMA is to design a microcontroller board that was small enough to fit into any project and low cost enough to use without hesitation.
The Attiny85 is a great processor because despite being so small, it has 8K of flash, and 5 I/O pins, including analog inputs and PWM “analog” outputs. The board has a USB bootloader so you can plug it into any computer and reprogram it over a USB port just like an Arduino (it uses 2 of the 5 I/O pins, leaving you with 3). Adafruit even made some simple modifications to the Arduino IDE so that it works like a mini-Flora. Arduino GEMMA - Miniature Wearable Electronic Platform is perfect for small and simple projects and very useful in wearable electronics projects.
Even though you can program Arduino GEMMA using the Arduino IDE, it's not a fully 100% Arduino UNO-compatible. There are some things you trade off for such a small and low cost microcontroller. Arduino GEMMA does not have a Serial port connection for debugging so the serial port monitor will not be able to send/receive data and USB v3 ports of some computers' don't recognize Arduino GEMMA ‘s bootloader. Simply use a USB v2 port or a USB hub in between
Arduino Web Site: https://www.arduino.cc/en/Main/ArduinoGemma
Adafruit Web Site: https://www.adafruit.com/products/2470