Azurite is a very popular mineral because of its unparalleled color, a deep blue called "azure", hence its name. The color is due to the presence of copper (a strong coloring agent), and the way the copper chemically combines with the carbonate groups (CO3) and hydroxyls (OH). Azurite has been used as a dye for paints and fabrics for eons. Unfortunately, at times its color is too deep and larger crystals can appear black. Small crystals and crusts show the lighter azure color well.