Larvikite

Larvikite forms part of the Larvikite–Lardalite Plutonic Complex in the Carboniferous–Permian Oslo Rift of south-eastern Norway; the Geological Survey of Norway dates it to about 290 million years ago, a time of strong volcanic activity in the Oslo Fjord region. It is widespread in Vestfold and Telemark, and the main quarry area for light larvikite is Tvedalen. Different intrusions yield different commercial types, including light larvikite ("Blue Pearl"), dark larvikite ("Emerald Pearl"), Malerød ("Royal Pearl") and Stålaker.

The blue iridescence is caused by optical interference in microscopic lamellae of alternating alkali feldspar and plagioclase within the ternary feldspar crystals. Because larvikite is a rock, a polished piece shows rhomb-shaped feldspar crystals with schiller set among dark grains of other minerals, whereas labradorite is a single plagioclase mineral; the two can look alike on a dark ground. Gemdat notes that informal names such as "blue granite" or "bird's-eye granite" are not accurate descriptions, since larvikite is not a granite.

The geologist Waldemar Christopher Brøgger (1852–1940) named larvikite after the coastal town of Larvik. Its first recorded use as building and ornamental stone dates to the 12th century, when many stone churches in the region were built of it; modern quarrying began around 1820 and grew into an export industry in the late 19th century, and larvikite won a gold medal at the 1886 Liverpool world exhibition. It has been used on buildings such as Harrods in London, the Bank of America tower in Jacksonville and the Burj Al Arab in Dubai, and for gravestones worldwide. It was chosen as Norway's national rock and was designated an IUGS Global Heritage Stone Resource in 2017.