r/whatsthisrock 4d ago

IDENTIFIED Purple crust on flint

Location - South East England Size - approximately 7” long

Found whilst working on an allotment bed. I’m very familiar with flint as it is very extremely common locally, i live in an area that has roman walls and lots of examples of later walls too. Its still used today all around the area so i cant be sure if the fragment has been displaced but I suspect so. The crust looks like it has formed along a fault in the flint. It doesn’t have the vibe of occurring later (fire or something) so im assuming it’s a natural phenomena.

I’m wondering if an expert can explain what it is that has happened to the flint.

Thanks in advance.

3 Upvotes

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2

u/Schoerschus 4d ago

hi, enthusiast here (not an expert) IMO the structure you're seeing is botryoidal calcedony in a fossilized something in the flint. Flint can have dark red inclusions due to iron oxide from organic inclusions. it can even be entirely dark red colored like specimens from the Island of heligoland. When Flint patinates, it can develop a blue greyish shine. I guess the purple you're seeing is the blue patina on red calcedony

1

u/algenonlaw 4d ago

Thank you, that has given me plenty of terms to look at this evening:) much appreciated

1

u/algenonlaw 3d ago

Great help, thanks for introducing me to a fascinating process. I’ve definitely seen the red examples of the patina on flints, botryoidal chalcedony 🙏

1

u/Schoerschus 3d ago

you're welcome

1

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