The most diagnostic property of azurite is it distinctive deep blue color. It is also soft with a Mohs hardness of only 3.5 to 4. It contains copper, which gives its blue color and a specific gravity of 3.7 to 3.9, which is exceptionally high for a non-metallic mineral. Azurite is a carbonate mineral and produces a slight effervescence with dilute hydrochloric acid, producing a light blue liquid. Azurite produces a light blue streak on unglazed porcelain.