Dear Mr. Marketing,

rock color is one of the most personal views you can have of a rock. You can describe the rocks e.g., by RGD or any other color code but it is difficult to give any profound answer unless you have further details concerning the mineralogical and chemical composition of or organic input into the surface expression/coating.

Best regards

H.G.Dill

Your question is a bit confusing...you said what makes rock turn?

do you mean... your are adding some chemicals or applying some staining technique by which the rock turns magenta.. or is it originally magenta?

Some clarity please.

It is just a surface coating. If I cut the rock in half the cut surface would be black.

Thanks for your response.

Paul

Geologist

North Pembrokeshire

Wales

The rock is black, fine-grained, un-layered, non-glassy.

The coating is 1 millimeter thick. 

Will try and get you a photo.

Thanks for getting back to me.

Paul

Photograph of the "Magenta" rock now attached.

Hi Mark.

Yes, I am sure it is not paint. Never seen paint this bright !!

Thanks Mark. I think it is organic - it is not the rock colour.

Are you suggesting a link to the pink flints of Blick Mead near Stonehenge?

http://www.judyhall.co.uk/miscellaneous/out-of-the-woo-woo-past-day-glo-pink-flints-and-stones-falling-from-the-sky

https://species.nbnatlas.org/species/NHMSYS0000603347

If you're thinking that it's Hildenbrandia Rivularis, then its distribution isn't limited to the clear chalk stream of the Hampshire Avon.