I met Michelle on Gemstone Beach in September last year (2024). She is an avid beach stone collector. I offered to tumble polish some of her stones. I posted her the first lot of 51 polished stones in mid-November – 24 of them are featured in this Post and this Post. I finished a second lot, 60 stones, in mid-November – 14 of them feature in this Post.
When I was in Riverton, going through Michelle’s stones to select the ones to take back to Whanganui to tumble, I came across one that particularly intrigued me. I spent a bit of time turning it over in my hand, trying to see what was going on in it. It kind of looks amygdaloidal, also known as a volcanic “filled bubbles” stone.
Bubbles of gas get trapped in the hardening volcanic rock, forming tiny holes called “vesicles”. These holes then fill with hot mineral-rich fluids which leave behind deposits of minerals such as quartz, chalcedony (agate), calcite and zeolites (zeolites are a group of minerals with a crystalline structure made up of silicon, aluminum, and oxygen). The resultant “spots” are called “amygdales” (also “amygdules”), a term that comes from the Latin and Greek words for almond, reflecting the almond-shape of many such in-filled vesicles.
Michelle said I could keep this stone for myself, and I accepted her kind offer. The stone turned out to be a bit rough, to have small indentations in it, in the amygdales. But I didn’t want to smooth the stone out too much in case it lost its character. So I just did the minimum with it, tumbling it in 220 grit, then 400 grit, then in tumble polish. The polished stone is now clearer but it still has small rough bits, especially in the amygdales.
I think the identification of the stone is confirmed – it looks to me that the pattern in it is of amygdales. The “indentations” and tiny rough areas are where the infill minerals (at least some of it looks like quartz) has failed to become as hard as the volcanic rock itself.
Thanks to Michelle for letting me have this interesting stone she found on Gemstone Beach.
