Azurite
Azurite, a dark-blue mineral used as an ore of copper. Azurite frequently occurs as well-defined crystals. It can be transparent to translucent, and has either a glassy or a diamondlike luster. When it is placed in acid, azurite effervesces and gives off bubbles of carbon dioxide gas. Azurite leaves a light-blue streak when rubbed against a piece of unglazed tile. High-purity azurite is polished and used for jewelry. At one time, azurite was widely used as a blue pigment.
Azurite is usually found in combination with various other copper-containing minerals, particularly with malachite. Some of the finest specimens of combined azurite and malachite come from Arizona.
Chemical formula: 2CuCO3(OH)2. Specific gravity: 3.8. Hardness: 3 12-4.