Nuummite

Nuummite has been found at eight localities within about 50 km of Nuuk, the capital of Greenland, in orthoamphibole layers and pods within Archean supracrustal rocks and gneisses (described as the Malene supracrustals, in the Akulleq terrane). Gemological studies describe the host rocks as more than three billion years old; material from Simiuttat island has been dated to about 2,732 to 2,707 million years. A further occurrence has been described at Kangerluarsuk in the Maniitsoq region. Mindat notes that some "pseudo-nuummites" come from China and India.

The iridescence is rarely visible on rough pieces and appears clearly only after cutting; most cabochons show one or two colors. In golden material from Simiuttat, transmission electron microscopy showed that it comes from interference of light reflected from alternating gedrite and anthophyllite exsolution lamellae with an average spacing of about 180 nm, a spacing that gives yellow iridescence. Cabochons smaller than 0.5 cm usually contain too few iridescent grains to look attractive. The rock fluoresces dark violet under ultraviolet light. Because pseudo-nuummites from other countries are on the market, the name alone does not establish a Greenland origin.

Orthoamphiboles have a hardness of about 6. In a test reported in Gems & Gemology, nuummite cabochons placed in a window in sunshine for more than a year showed no change in appearance. Fresh material is generally massive without pervasive cracks, but material from near the surface tends to have small cracks.

Iridescent orthoamphibole from the Nuuk area was first collected by K. L. Giesecke in 1810 and described by O. B. Bøggild in 1905 and 1924. The geologist Peter W. Uitterdijk Appel of the Geological Survey of Greenland rediscovered it as a gem material in 1982. The Nuuk town council gave it the trade name Nuummite, a company owned by the Nuuk municipality received an exploration concession and permission to make and sell jewelry, and the stone was launched on the Greenlandic market before Christmas 1986.