Spectrolite

Spectrolite comes from the Ylämaa area of Lappeenranta in south-eastern Finland, where the highest-quality deposits occur. There it lies in six anorthosite bodies (leucogabbronorite, leuconorite, leucogabbro and anorthosite) enclosed in the 1615–1646 million-year-old Wiborg rapakivi granite batholith; the Ylijärvi anorthosite is dated to 1633 ± 2 million years. The main quarries are in the villages of Ylijärvi, Mättö and Pahaoja. In the stone industry the spectrolite rock is a coarse-grained black to dark gray stone with iridescent crystals 0.5 to 15 cm across, sold as Spectrolite, Gem Spectrolite, Noble Spectrolite, Arctic Blue, Finlandia Blue and Ballado Blue. Iridescent labradorite also occurs elsewhere in Finland, for example at Jaala.

Spectrolite is labradorite, so it shares labradorite's hardness, density and cleavage; what sets it apart is the intensity and range of its colors on a dark ground. The colors are caused by interference of light reflected from extremely thin lamellae in the feldspar: areas with thicker lamellae look red and areas with thinner lamellae look blue. Strictly the name belongs to Finnish material: the Finnish natural-stone report restricts the name to Ylämaa material and Mindat records it only from Ylämaa, while Geology.com notes that strongly colored labradorite from other places is often also called spectrolite in the trade, so the name alone does not prove a Finnish origin.

Labradorite has a Mohs hardness of 6 to 6.5 and two directions of perfect cleavage, so it can break from a sharp knock and scratches more easily than harder gems such as sapphire or agate; Geology.com advises caution in jewelry exposed to impact.

Spectrolite was found at Ylämaa in the 1940s and quarrying began in the 1950s; according to the Geological Survey of Finland it has gradually become Finland's best-known and most important gemstone. Geology.com reports that the name was given by the director of the Geological Survey of Finland. The rock has been used for gemstones, small objects, gravestones, monuments, interior decoration and kitchen tops, and was exported to countries including Italy, China, Japan and the USA. Spectrolite was chosen as the provincial stone of South Karelia in a public vote organised in 1989.