Geologically speaking, nuummite is of volcanic origin and was formed about 3 billion years ago.  Subsequent influences on the rock (metamorphism) have given rise to the striking mixture of crystals which gives nuummite its unique appearance.  Rocks resembling nuummite are also found in a few minor occurrences in the USA, but it is only in the Greenland type that coloration is developed well enough for the stone to be suitable for gemstones.

In nuummite, the two orthoamphibole, anthophyllite (Mg,Fe)₇Si₈O₂₂(OH)₂ and gedrite (Mg,Fe)₅Al₂(Al,Si)₈O₂₂(OH)₂, both with a hardness of 5 to 5.6, constitute a mixture of elongated crystals, often in sheaf-like groups.  In the transition between the individual crystals (and especially the thin ones), an optical effect is created causing a special “inner” golden brown glow.  This effect is also called iridescence, and is especially distinct on polished surfaces. The result is that the crystals appear as bright lamellae, almost like flames in a fire.  The colors vary somewhat between reddish, greenish, and bluish hues, sometimes even within the same lamella.  Between the bright lamellae, the color is dark brown to black.

Nuummite is generally easy to polish, even though, in certain qualities of stones with many parallel crystals, it can be difficult to avoid holes and cracks.  The usual shape is cabochon, but other convex finishes also produce pleasing results.  In larger pieces it is possible to retain most of the colors of the iridescence, so that one end of the cabochon has a golden hue while the opposite end has a bluish tone.  Nuummite is well suited to mounting in both gold and silver.

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