Vivianite

Vivianite crystallises in elongated prismatic or tabular forms or in lamellar or fibrous aggregates, sometimes forming crystals even more than a metre long.

When fresh it is colourless or, more rarely white, but at the slightest alteration it immediately begins to darken, changing from light blue to dark blue and even to green-blue.

It originates in the oxidation zones of iron and manganese deposits.

It is used in large quantities in the dye industry, in addition to the more frequent interest of collectors.