Are shells allowed in my luggage?

For the sake of the rules of this sub I am a current resident of Japan.

So I’m visiting my home country (the UK) this week, I wanted to take back some very small things I found beachcombing. Things like small stones, bits of sea glass, and a couple of little shells. The shells have nothing in them, just washed up on the shore, they’re pretty small too.
I’ve searched online and I see no mention of shells at all regarding customs and whether they’re allowed in luggage, can anyone here shed any light on this? Thank you