Malachite and azurite: copper minerals that are literally connected

Malachite and azurite are both copper carbonate minerals and are frequently found together in the same deposits, often in the same specimen, because azurite can actually alter into malachite over geologic time through a genuine chemical transformation called pseudomorphism, where one mineral replaces another while keeping the original crystal shape. Malachite is green with distinctive banding, azurite is a deep blue, and both are relatively soft at around 3.5 to 4 on the Mohs scale. Both will react with dilute acid similarly to other carbonates, though testing should be avoided on display quality pieces since it will visibly etch the surface.