Lapis lazuli is a beautiful blue stone with a long history, having been mined in Northeastern Afghanistan for over 8,000 years. Although it can be found elsewhere - Chile, Pakistan, Russia and Mongolia, mostly - Afghanistan is still be far the world's biggest supplier. It has a complicated chemical make-up, but the important thing you need to remember is that it's blue. Very, very blue.
These are tumblestones, roughly 3m across.