This is a slightly expanded version of my nineth contribution to the current Alphabetical Series in the Facebook Group “New Zealand Lapidary, Rocks, Minerals, Fossils”. In the 2021 version of this Series I posted “I is for Ichnogenus Protovirgularia”, a trace fossil stone.
I is for Intrusive Igneous stone.
I quite like the black-and-white spotted stones I find on Gemstone Beach and elsewhere along the Te Waewae Bay coast. They are not breccia (made up of fragments or clasts) and are not amygdaloidal (infilled gas bubble holes), but have tiny white crystals. The size, shape and distribution of the white crystals can vary considerably, as illustrated with Stones W43 to W61 in this Post. And they can, of course, be found in other parts of New Zealand. I found this one on 3 August this year on Gemstone Beach. Stones like this are likely to come from igneous rock that has cooled while being below the surface of the Earth – this is also known as plutonic, after Pluto, the Roman god of the underworld. Its solidification takes a long time so crystals have time to grow bigger, large enough to be seen with the naked eye. By contrast, in extrusive volcanic rock which cools a lot more quickly, crystals have little time to grow and are often visible only under a microscope. See Geology.com for a good introduction to igneous rocks. The commonest intrusive igneous rock is granite. Others include gabbro and diorite (see the diagram at the end of University of Auckland Geology) which are said to occur on the Te Waewae Bay coast but I have not yet worked out how to identify them.
The next entry is “J is for Jasper”. An Index for this Alphabetical Series is here.
I don’t think I’ve ever found a gabbro or diorite one either! I do like those black and white fleck stones too!