Spectrolite
Spectrolite is an uncommon variety of labradorite feldspar.
Spectrolite exhibits a richer range of colors than labradorite (that shows only tones of blue-grey-green) and high labradorescence.[1][2] Spectrolite was initially a brand name for material mined in Finland, but is sometimes incorrectly used to describe labradorite whenever a richer display of colors is present, regardless of locality: for example, labradorite with the spectrolite play of colors has also reported from Madagascar.[1] The difference between Finnish spectrolite and other labradorites is that crystals of the former have considerably stronger colourfulness than other labradorites, caused by the black base color of spectrolite feldspar; other labradorites have mostly a transparent base color. Spectrolite is often cut as a lapidary cabochon, similar to plain labradorite, to enhance the effect and is used as a gemstone.
Finland deposit
Finnish geologist Aarne Laitakari (1890–1975) had described the peculiar stone and sought its origin for years when his son Pekka discovered a deposit at Ylämaa in south-eastern Finland while building the Salpa Line fortifications there in 1940. The Finnish stone exhibits a uniquely vivid iridescence and a full spectrum of colors, hence the name "spectrolite" was coined by the elder Laitakari. The name spectrolite is sometimes incorrectly applied to any labradorite of similar colours.
The quarrying of spectrolite began after the Second World War and has become a significant local industry. In 1973 the first workshop in Ylämaa began cutting and polishing spectrolite for jewels.
References
Seppo Lahti I.1989 The origin of interference colours in spectrolite (iridescent labradorite).Geologi 41.