Arfvedsonite is the iron rich member, while the magnesium rich member is called, unimaginatively, magnesio-arfvedsonite . The two minerals are almost indistinguishable by ordinary means. All of these sites contain agpaitic pegmatites and nepheline syenites. Color is greenish-black, dark gray or black. Luster is vitreous to dull or silky in fibrous forms.
Crystals are translucent to opaque.
Crystal Habits include prismatic to acicular or even tabular crystals sometimes in radial stellate aggregates. Some specimens are up to 20 cm in length. Also fibrous, lamellar and massive. Twinning is seen as lamellar twins. Fracture is uneven to subconchoidal.