However, with a increase in iron percentages the color darkens, the streak is bluer, the density increases and the luster increases. Its presence in igneous rocks is quite different from glaucophane which is restricted for some reason to metamorphic rocks. Asbestos riebeckite is only found in metamorphic rocks, however. Color is usually dark blue to black. Luster is vitreous or silky in fibrous forms.
Crystals are translucent.
Crystal Habits include slender prismatic to acicular, often aggregated crystals, columnar, granular and fibrous, asbestiform masses. A moss-like aggregate habit has been described when found in some igneous rocks. Fracture is splintery to uneven. Hardness is 5 - 6. Streak is blue-gray.