Stay-at-Home Day Sixteen, Good Friday 10 April 2020: Stone Sixteen

Stone Fifteen.

Stone Fourteen.

Stone Thirteen.

Stone Twelve.

Stone Eleven.

Stone Sixteen is a reasonably common stone on Gemstone Beach. That is, I often see  stones like it when walking there. 

Stone Sixteen.

Stone Sixteen.

Stone Sixteen.

Some of the stones on Gemstone Beach that look similar vary a lot in grain size and may actually be composed of different material. But they have the same black and white mix of grains or crystals.

Other polished black and white grained stones from Gemstone Beach in my collection.

Stone Sixteen.

Stone Sixteen is oval and flat. It has polished well. I initially thought of it as a predominantly black stone but a detailed examination of it shows plenty of white grains.

Stone Sixteen.

The make-up of Stone Sixteen.

The make-up of Stone Sixteen.

The make-up of Stone Sixteen.

Stone Sixteen.

Stone Sixteen up close.

I am not sure what kind of stone Stone Sixteen is. My best guess at present, based on a little research done today, is that it could be either gabbro or dolerite. Both are igneous plutonic rocks and occur in the Gemstone Beach area or the wider Southland region. According to the University of Auckland geology website, gabbro is a dark grey to black colour. It is often found along mid-ocean ridges or in ancient mountains composed of compressed and uplifted oceanic crust. Gabbro is the “plutonic” equivalent of basalt, that is, it forms under the surface of the earth while basalt forms above the surface. It is composed mainly of the minerals plagioclase (light) and augite (dark). Dolerite is a fine- to medium-grained, dark gray to black igneous rock, also plutonic. It is extremely hard and tough. It is said to be compositionally equivalent to gabbro and basalt but texturally between them. In other words, dolerite is in effect a finer-grained form of gabbro. It is a significant part of the Livingstone Mountains, just to the north-east of Gemstone Beach.

The Riverton Museum “Te Hikoi” has examples of gabbro and dolerite in its geology collection. The gabbro stone comes from Monkey Island which is just a couple of kilometres to the south-east of Gemstone Beach. “A Photographic Guide to Rocks and Minerals of New Zealand” notes on page 73 that gabbro can be seen between Colac Bay and Pahia, the part of the coast just past Monkey Island. The dolerite stones in the Riverton Museum are noted  as from Gemstone Beach. 

Gabbro stone, Riverton Museum, photo taken by me in 2016 with museum staff permission.

Dolerite samples, Riverton Museum, photo taken by me in 2016 with museum staff permission.

The next Post in this Series is Stay-at-Home Day Seventeen, Saturday 11 April 2020: Stone Seventeen.

Author: tumblestoneblog

Retired Academic, male, living in the New Zealand countryside near Whanganui with his wife as well as Jasper the dog, Fluffy the cat, Dancer and Penny, the horses, and a shed half-full of stones. Email john.tumblestone@gmail.com. View all posts by tumblestoneblog