I am a member of the Facebook Group “New Zealand Lapidary, Rocks, Minerals, Fossils”. In 2021, the Administrators invited members to submit photos each week of a stone, mineral or fossil in their collection that started with a letter of the alphabet. I posted my entries on TumbleStone Blog – see here for the Index to them. Recently, some members of the Group thought it would be a good idea to do it again. Here is my first entry (slightly revised and extended) for the 2025 Series. (In 2021, I posted “A is for Amygdaloidal”.)
“A is for Agatised Fossil Bone”
I found this stone on a beach near Kakanui, North Otago, in September 2024. I had not seen anything like it before, nor since. It has been suggested that it is agatised fossil bone. You can view similar material if you google “dinosaur gembone”. The process that produces it is called “permineralisation”, where the empty spaces within a buried organism, spaces filled with liquid or gas during life, become filled with mineral-rich groundwater. Minerals precipitate from the groundwater, occupying the empty spaces. This process is well described in Geologyin.com (though sometimes the ads can be annoying on this website). I thought about whether to tumble polish the stone for a while and eventually decided to do so.
I tried some gentle tumble polishing: a couple of short tumbles in 400 grit, then a 600 grit stage followed by a polish powder tumble. I did not have spectacular success (see photos above) – it looks like the material varies significantly in hardness, parts of it resisting taking a shine.
NOTE: A couple of members of the group, from the local area, mentioned they had similar though non-agatised stones, identified as fossil whale bone.
My next entry in this Series is “B is for Banded Argillite”. An Index for this Alphabetical Series is here.
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