Name: Azurite
Derivation: From "lajward" (Persian) blue
Formula: Cu3(CO3)2(OH)2
Description: Azurite is dark blue. It can have flat crystals. You can see layers of them to the left, and the specimen on the right is a single crystal. 

You usually associate "azure" with being pale blue, like the sky, so it may seem odd that such a very dark blue stone has this name. However, if you rub a mineral across something, the colour that gets rubbed off is called the "streak". It's not always the same colour as the stone itself. This colour sometimes helps to identify the mineral. Azurite may be dark blue, but its streak is pale blue, and if you grind it to a powder, it makes a pale blue paint pigment, for painting skies!