This is a slightly expanded version of my letter “Z” contribution to the current weekly Alphabetical Series in the Facebook Group “New Zealand Lapidary, Rocks, Minerals, Fossils”. In the 2021 version of this Series I posted “Z is for Zoomorphic Shape in Stone”.
Z is for Zeolite
This is an amygdaloidal stone (unpolished) I found on Gemstone Beach in August 2025. I have found similar ones there before – they are dark red, likely to be basalt, with “spots” of interesting colours. Here is a smaller tumble-polished specimen:
The spots are in-filled gas bubbles and the infill is likely to be a zeolite mineral. Molten magma often contains dissolved gas which can form bubbles in the rock as the pressure is released on eruption. These bubbles can get trapped in the solidified rock, forming tiny holes called “vesicles”. These holes then fill with mineral-rich fluids which leave behind deposits of minerals such as quartz, chalcedony, calcite and zeolites. The resulting in-filled holes are called “amygdales” or “amygdules” from the Latin and Greek words for almonds because of their often almond-like shape. However, amygdales can also be round and some can be irregular shapes, as can be seen in this Post’s two Gemstone Beach stones and the specimens below:
To get back to zeolite: In the 18th century, Swedish mineralogist Axel Fredrik Cronstedt first identified and named zeolites when he observed a group of minerals which released large amounts of steams when boiled – the name “zeolite” is derived from the Greek words “zeo”, to boil, and “lithos”, stone (Wikipedia and GeologyScience). There are more than 40 naturally occurring zeolite minerals, including natrolite, stilbite, phillipsite and heulandite. The zeolite wairakite (photo below) was first discovered and named in New Zealand in 1955 by Alfred Steiner, a pioneer geothermal scientist. He had found it while examining hydrothermally altered drill cores at Wairākei in the course of exploration for geothermal steam (Te Ara). It has subsequently been found worldwide.
Zeolite minerals have a wide range of applications in diverse industries, thanks to their unique structural and chemical properties. Their ability to adsorb (to hold onto molecules as a thin film), exchange cations (positively charged particles), and act as molecular sieves has made them invaluable in many industrial and environmental processes, such as water purification and wastewater treatment. The letter “Z” featured below is formed by small thumb-nail size tumble-polished Gemstone Beach stones, including four red amygdaloidal stones.
That is the end of this Alphabetical Series. An Index for the Series is here.
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