TumbleStone Calendar 2026: June – Black & White Igneous

The Introduction to this Calendar can be found here. 

The month of June in the 2026 TumbleStone Calendar features six black and white stones, all found on the shores of Te Waewae Bay, Southland, five on Gemstone Beach and one a little further west on Papatotara Beach. These stones are all igneous. “Igneous” rock forms through the cooling and solidification of magma or lava. There are two kinds of igneous rocks. The first is “volcanic” (also called extrusive”), having come from volcanoes and therefore solidified relatively quickly. The second is “plutonic” (also called “intrusive”), having formed underground from molten magma and therefore solidified relatively slowly. The white in these igneous stones are crystals that have formed as the rock has cooled – the longer it takes for the rock to solidify and cool, the larger will be the crystals. I find it difficult to identify the type of rock each stone below comes from, and the identity of the white crystals likely varies from quartz to feldspar, among others. The black minerals could be hornblende or augite or even olivine. See the end of this Post for a slightly more detailed discussion on identification issues.

I found Stone One (top row, left, in the Calendar) on 3 August 2025:

Along with Stone Three, it has the tiniest crystals of the six stones, although some of the white specks seem to be clusters of crystals.

Stone Two (top row, centre, in the Calendar) was found on Papatotara Beach on 16 August 2025:

Its crystals are larger and more obviously rectangular, although again there seems to be a lot of clustering of them. They look to me to be plagioclase feldspar. A very similar stone has been labelled diorite in the collection board of long-time Gemstone Beach fossicker, Marion Troon, which featured in a Guardian article on the beach in September 2024:

Stone Three (top row, right, in the Calendar), found on 12 March 2025, has a slight green tinge to it:

There are some black areas in Stone Three as well as some very fine black veins. As in the other stones, close observation reveals that the crystals within it vary significantly in shape.

Stone Four (bottom row, left, in the Calendar), found on 1 August 2025, has, along with Stone Six, the largest of the crystals:

However, there are a range of crystal sizes in Stone Four, some being quite small, and again there is a diversity of crystal shapes. from square to triangular to rectangular to circular.

Stone Five (bottom row, centre, in the Calendar) has an unusal “smear” that runs from its middle to the lower right, a distortion of its crystals, almost as if a finger was rubbed along it while it was in the process of cooling:

I found Stone Five on Gemstone Beach on 18 August 2025.

Stone Six (bottom row, right, in the Calendar), found on 23 March 2025, has the least number and most widely dispersed crystals of the six stones:

Altogether, these six June stones illustrate how black stones with white spots are not all the same. Far from it, a great diversity exists. For even more specimens, see the 21 stones in Section 5 “Stones with Opaque White Crystals” in this Post in the Series “Gemstone Beach and its Stones: An Introduction for the Passing Motorist”.

A NOTE ON IDENTIFICATION ISSUES: Geological books and websites describe different volcanic and plutonic rocks but I find that working out which best fits a particular stone can be difficult. Take, for example, gabbro and diorite, both plutonic rocks. Their descriptions in the book “A Photographic Guide to Rocks and Minerals of New Zealand” (Mortimer, Campbell and Low, 2011) set out the differences as follows. It is noted there that gabbro and diorite can be very alike in the field but diorite is a little lighter in colour and has a speckled salt and pepper appearance. Gabbro consists of about 50% to 90% dark minerals, often augite or olivine, the rest being light-coloured plagioclase feldspar. Sometimes gabbro can have a greenish colour due to the presence of chlorite. Diorite consists of about 25% to 50% dark minerals (usually hornblende), the rest being plagioclase feldspar with minor amounts of quartz or orthoclase feldspar. The authors note that gabbro can be found between Colac Bay and Pahia in Southland, which is just a handful of kilometres east of Gemstone Beach. Diorite, they write, can usually be found in the same places as gabbro. So the six June stones could reasonably be gabbro or diorite. Photographic comparisons of gabbro and diorite on websites don’t always aid in distinguishing the two types of stones. When I consult University of Auckland Geology, gabbro looks distinctly darker than diorite, as expected (first two photos below) but other websites often cloud the issue, showing a variety of different types of rocks, even with different crystal shapes and sizes.

In regards to a specific stone found on a beach, it can be difficult to decide whether it might be gabbro or diorite (or even something else).

For July’s trace fossil stones, see this Post. These 2026 Calendars are for sale, with postage within New Zealand included, for $35 non-rural, $40 rural. Email me at john.tumblestone@gmail.com.