Riebeckite

Riebeckite is a sodium-rich member of the amphibole group of minerals, chemical formula Na₂(Fe,Mg)₅Si₈O₂₂(OH)₂. It crystallizes in the monoclinic system, usually as long prismatic crystals showing a diamond-shaped cross section, but also in fibrous, bladed, acicular, columnar, and radiating forms. Its hardness 5.0 - 6.0, and its specific gravity is 3.0 - 3.4. Cleavage is perfect, two directions in the shape of a diamond; fracture is uneven, splintery. It is often translucent to nearly opaque.

It typically forms dark-blue elongated to fibrous crystals in highly sodic granites, syenites, iron formations and schists.