Metamorphic
One of the oldest gem materials on earth at roughly three billion years — a black amphibole rock threaded with iridescent gold, green and violet flash from intergrown crystal lamellae. Mined by hand from a remote Greenland site with a short seasonal window.
Flash appears as elongated needles or flecks that light up one at a time as the stone turns, not as a broad sheet like labradorite. Beware 'nuummite' from other localities, which is generally duller astrophyllite or arfvedsonite rock.
Reasonably hard but the polish is the whole value. Avoid abrasives and ultrasonics.
Greenlandic Inuit collected it long before European description in 1810; export is now regulated and supply is genuinely limited.
Cultural history, recorded as tradition rather than as medical or scientific claim.
See it in person
Both rooms are open six days a week, and there is always someone happy to pull pieces out of the case so you can compare them under the light.