Weathered rocks from the beach lol. These things came from the reef just outside the heads.

There was a massive wind event about 2 yrs ago that kicked up so many of them (some of them huge) so carted them back to put on the windowsill. Problem is you have to keep taking them off and cleaning it as spiders and dust loves it.

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Sill is about 5m long and completely full of them. Always look at them, zone out and think of the beach.