Both Azurite and Malachite have been used as pigments since ancient times. If the stone is pure, the color holds true. If however, the rock that is used is in the midst of a transformation, Azurite turning into Malachite, the pigment won’t hold a pure color long-term.
In this oil painting, The Entombment of Christ by Michelangelo, the dress of Mary Magdalene (figure on far right) was painted using Azurite pigment. Originally the dress was a lovely shade of blue, but it has now faded to olive green. This is because Michelangelo’s Azurite wasn’t pure enough, it had too much Malachite mixed in.