Treasure?

I found this stone, or half stone on the beach and it seemed obvious that it had had a crystalline structure of some sort in the cavity. You may be able to see the glitter from remaining glitter.

I assumed the sea had broken it, and spent some time looking around for the other half. Then I looked for similar stones - I found one easily enough, and cracked it open easily - but alas, there was no treasure inside.

I was telling a friend's father about it later, and he suggested that there had been human intervention. I hope to spend more time looking and seeing if I can find my own treasure.