A friend of mine found these while diving in Australia and we have been trying to figure out with they are but have no idea and out internet searches aren't helping. Any guess?

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Since we don't know let me chuck in a couple of guesses.

Could they be cuttlebones, the internal shells of cuttlefish?
https://en.wikipedia.org/wiki/Cuttlebone

Or could they be tests of some kind of sea-urchin?
https://en.wikipedia.org/wiki/Sea_urchin#Endoskeleton

I doubt they are actual bones.