Hitchhiker ID Please

[mortigas]

Got some zoanthids over the weekend and I noticed these pyramid-shaped hitchhikers (circled in yellow) attached to some zoa polyps. Initially, I thought that they were grains of sand, crushed coral, starfish arm, or pyramid snail; however, they do have a completely symmetrical, 3-dimensional pyramid form. Each pyramid is quite hard, has pointed tips and little white specks on the surface (though my camera can't show them clearly). The sizes range from 1-2 mm.

I have searched the net but can't find anything on this. Can you help identify these mysterious hitchhikers? Is it animal or mineral?

Thanks.

[Monti]

Do they move?

[mortigas]

No, they don't seem to move. Their pointed tips are sharp and those keep them lodged to the zoa polyp top or sides. Even a freshwater dip and swirl did not dislodge any. I was just surprised to see them the following day. I removed some via the turkey baster to examine closely.