The Introduction to this Calendar can be found here.
The month of August in the 2026 TumbleStone Calendar features six yellow stones of various hues. They come from five different South Island beaches. Yellow in a stone is often produced by the presence of iron oxide, which can also give rise to browns and reds and even blacks. Yellow is sometimes due specifically to the presence of hydrated iron oxide hydroxides, also known as limonite. Mindat notes that the most common form of limonite is goethite, but it can also include hematite, for example, and even some quartz and clays.
Stone One (top row, left, in the Calendar) comes from Ward Beach in Marlborough, found on 29 July 2025, the day on which I also found September’s unidentified Stone Four. It is a yellow stone with a white vein and dark markings which look like lines of dots:
The stone could be limestone, which is common on Ward Beach. Limestone is usually a biological sedimentary rock, forming from the accumulation of shell, coral, algal, and other organic debris – see Geology.com and “Selection of Tumbled Ward Beach Stones: Part 2 – Other Limestone Pebbles”. I am unsure what the white vein consists of as it does not have the usual density and brightness that quartz would have. The black lines are most likely dendrites which are are thin, branching crystals, often of some variety of manganese oxide (or iron), that grow over a surface of a rock. They are usually found in cracks or along bedding planes. See “D is for Dendrites” (the second part of this Post) and the second part of “Selection of Tumbled Ward Beach Stones: Part 3 – Stones With Veins; Stones With Dendrites”.
Stone Two (top row, centre, in the Calendar) is a hydrogrossular garnet collected from Gemstone Beach, Southland, on 3 August 2025:
Hydrogrossular garnets come from dense rock masses, not crystals. The first ever identification of hydrogrossular garnet in the world was in 1943 by New Zealander Colin Hutton, from stones found in Nelson. Many fossickers visit Gemstone Beach to look for them as they are easily polished and make excellent pendants. Hydrogrossular stones often feel waxy and some are partly transparent. “Grossular” derives from the Latin word for gooseberry, referring to the light-green colour of some hydrogrossular garnets, and some are a light grey. However, other minerals can get mixed in, as in the case of Stone Two where something like iron oxide has produced its yellow-orange patches. Stone Five (below) is also a hydrogrossular garnet and the month of December in this Calendar features five more hydrogrossular garnets.
Stone Three (top row, right, in the Calendar) comes from Bluecliffs Beach, to the west of Gemstone Beach along the Te Waewae Bay coast:
I found Stone Three on 20 August 2025, one of only nine I collected that day – as you move west along this coast, the stones get bigger, so Bluecliffs Beach has mainly quite large ones. I mainly adopted a “photo fossick” approach with the larger stones, coming away with only a photograph of them. The smaller stones were much less common. Stone Three is probably an iron oxide-stained quartz, but it has a bright yellow-orange hue unevenly distributed through it, leaving interesting patterns.
On a beach near Slope Point at the very bottom of the South Island, I came across Stone Four (bottom row, left, in the Calendar) on 10 March 2025 – it’s yellow with some flecks of red:
This was quite a surprise as I usually collect this type of stone on Seadown Beach, Kakanui, North Otago, nearly 300 kilometres north of Slope Point. Stone Six is such a find from Kakanui. What is special about these yellow-brown stones is that they contain lots of tiny fossils, most of them bryozoa. Bryozoa are one-celled animals which live in colonies. They have a partially calcified hard body wall, which is subject to the process of fossilisation. There are many species and many different types, including branching twig-like forms, lacy forms and fan-like forms. It is very difficult to see them with the naked eye – the close-up photos reveal the complexity of them. I am grateful to John Taylor, who lives in Scotland, for help in identifying this type of stone, who also suggested it was chalcedony (a form of cryptocrystalline quartz) – see “Bryozoan Fossils in Chalcedony, Kakanui’s Seadown Beach”. There is a little bit of clear or white chalcedony that can also be glimpsed in some of the veins in Stone Four. Slope Point beaches are usually dominated by rhyolite stones, with some nice petrified wood there too, but every now and then I come across a real surprise, like Stone Four.
Stone Five (bottom row, centre, in the Calendar), like Stone Two, is a Gemstone Beach hydrogrossular garnet, found on 12 August 2025:
In Stone Five, the iron oxide (if that is the mineral providing the colour) is through all of the stone and ranges in hue from light yellow to dark red.
Stone Six (bottom row, right, in the Calendar), like Stone Four, contains bryozoan fossils in chalcedony, though it was found 300 kilometres away at Seadown Beach, Kakanui, on 25 August 2025:
Compared to Stone Four, Stone Six has more brown in it, alongside the yellow.
For September’s set of “unidentified” stones, see this Post.
