The pebble

I have a huge bowl full of pebbles and shells collected from various beaches over several decades. (I've never grown out of the habit of putting stones and other interesting things into my pockets... .)This is the latest, a pebble picked up last week from the beach near Blakeney Point. The dark parts of it are actually blue, ranging from royal blue to navy, but neither the Lumix camera nor the phone camera capture its true colour. The white lines are very slightly indented - maybe some sort of thin worm fossils? I've no idea really, and cannot find images of a similar pebble on the internet.
It's interesting, and quite pretty - a nice addition to the collection.