Thank you everyone for your suggestions about my green pebble.
One suggestion comes via Sarah Waters who serendipitously read my post just before her brother-in-law, a geologist, came to visit. He says: ‘This is possibly a piece of Cretaceous Green Sandstone. The green colour is from the mineral glauconite which forms in shallow marine environments. This is quite a dark one. It is about 100 million years old and was formed at the height of the dinosaurs dominance of the planet. It almost certainly was washed into the sea at Hunstanton, where a thin out-crop reaches the sea, and moved along the coast by a process of long shore drift to Cley.’ So thank you Sarah for passing on the information.
Even more, she sent me a map of the geology of Hunstanton that shows where the pebble would have come from. It probably originated from the bright dark green area down the right hand side of the river at Hunstanton. See the map below.
So I think that little mystery has been solved. It has pleased me no end to get all your suggestions, and I have made this special pebble a little bag out of a fold of waxed silk that is just translucent enough to see its form and to glimpse a hint of its colour. It will live with the growing number of other found objects and their made containers that are presently multiplying down in the studio!