Johannite is a rare uranium. It crystallizes in the triclinic crystal system with the chemical composition Cu2·8H2O. It crystallizes in the triclinic system and develops only small prism or thin to thick tabular crystals , usually occurs as flaky or spheroidal aggregates and efflorescent coatings. Its color is emerald-green to apple -green and its streak is pale green.
Johannite is a strong radioactive mineral with a calculated activity of 87,501,143 Bq /g (to the comparison: natural potassium : 31.2 Bq/g).