Ooooh, pretty!!

This is what happens when a rock (grey) gets a crack, which then fills with fluid that crystallizes into a mineral vein (white), which unevenly weathers (shape + orange). https://twitter.com/beneth42/status/1289910659191205889

Without exact location, best guess is grey host rock is compressed ocean mud (slate) & the white vein is quartz (possibility calcite if it bubbles in acid), with rusty trace iron.

From shape, grey is older (had to form be fore it could crack!) & softer (eroded/rounded more).

With context comes corrections:

I’ve never pet Irish rocks (one day!), but from location & geologcal history ( https://www.gsi.ie/en-ie/geoscience-topics/geology/Pages/Geology-of-Ireland.aspxis), it’s likely calcite (lots of limestone!) & shale (less squashed than slate). https://twitter.com/beneth42/status/1290020102302203905

Aside:
If you’d like to learn more about metamorphic rocks (like the difference between shale & slate), you’ll want this thread: https://twitter.com/mikamckinnon/status/1232038309716615168

Q: Forbidden macaroon?

A: Nah, you can lick it. It’d be uninformative to taste unless you get salt spray from the ocean, but not harmful. Boring But Safe to Lick list.

Cronching would hurt your teeth, tho.