RHOMB PORPHYRY - Norway

Rhomb-porphyry is an igneous rock with rhomb-shaped, or lens-shaped crystals of feldspar embedded in a fine-grained matrix.

It is a volcanic lava that is rare globally, but it is most well-known from lava flows near Oslo in Norway. It only occurs in two other places on Earth – the East African Rift Valley and Mount Erebus in Antarctica. The flows are associated with current or former continental rifting.

The lava flows in the Oslo Fjord area cover thousands of square kilometres. This volcanism occurred almost 300 million years old and may have lasted as long as 20 million years. The magma that produced this lava slowly crystallised into a different type of rock known as larvikite, which is a popular decorative stone.

This is a very nice specimen. It is a thin, unpolished piece that stands up for display.