I recently presented 12 tumble-polished stones to Tony in appreciation for his support when I undertook work with him in the dairy industry. Part One of this series of Posts described Stones #1 to #3. This Post deals with the characteristics and origins of Stones #4, #5 and #6, and later Posts will examine Stones #7 to #12.
These are the 12 stones:
4) Stone #4 Pink Granite
This stone was found at Riverton in February 2018. Granite is a coarse-grained igneous rock, made up of quartz, feldspar and mica. Its grains are large enough to be seen by the unaided eye. Pink Granite is given its colour by a larger proportion of potassium feldspar. Granite is found exposed at various places on Stewart Island and the western part of the South Island. It can be seen along the coast between Colac Bay and Orepuki in Southland, and stones of Pink Granite can often be found on southern beaches. Click on the photos below to see their captions.
5) Stone #5 Finely Banded Rhyolite(?)
This stone was found at Orepuki, Southland, on Gemstone Beach in April 2016. I have collected a number of such stones before on southern beaches and find them fascinating as they conjure up the image of cosmic gas trails in faraway starfields. A fellow stone collector once told me they were Rhyolite but it is difficult to confirm this from the various online and published sources on Rhyolite, maybe due to the distinctive pattern on these particular stones.
Rhyolite is very similar in chemical composition to Granite. But while Granite has crystals that are generally easy to see, in Rhyolite the crystals are often too small to see. This is due to the more rapid cooling of the Rhyolite lava at the earth’s surface compared to granite’s slower cooling magma within the earth.
Examples of other Rhyolite stones found at Orepuki or Riverton:
Orepuki is a small village on a sparsely settled part of the southern coast. Gold was mined in the district during the last two decades of the nineteenth century, and in the 1880s the southernmost Chinese settlement in the world was to be found there. The beach just to the north of Orepuki is called Gemstone Beach – it is said that semi-precious gems such as garnet, sapphire, jasper, quartz and nephrite can be found on the beach, along with hydrogrossular stones and argillite stones containing fossil worm casts. (The garnets, sapphires and nephrite are extremely rare.) Gold can also be found in patches of black sand at low tide.
Location of Orepuki (source: Google Maps):
6) Stone #6 Marble(?)
I found this stone also on a Riverton beach. I don’t know what type of stone this is, but it reminds me of Marble. Marble is formed out of limestone which is subjected to the heat and pressure of metamorphism. It is composed primarily of the mineral calcite and in its pure form is white. However, it can also contain other minerals, such as clay minerals, micas, quartz, pyrite, iron oxides, and graphite, which provide colouring. Veined and patterned Marble, as this stone could be, is often created when a pure white original Marble is cracked or shattered and the spaces between the fragments are filled with other materials.
Stones #7 to #9 are described in Part Three.
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