Annabergite, Nickel Arsenate,
Formula Ni₃(AsO₄)₂.8H₂O)

Crystal system: Monoclinic (crystals are rare)
Hardness: 1.5 to 2.5
Density: 3.1
Colour: Apple green
Streak: Pale green to white

Annabergite is a rare secondary mineral formed by oxidation of cobalt and nickel arsenides and sulphides in hydrothermal deposits.

It is sometimes described as an apple green variant of Erythrite. Erythrite is a purple colour and is cobalt arsenate.

The mineral is named after Annaberg in Germany where it was originally found. The specimen in the picture came from Lavrion in Greece.