Data: This small rock was found on a large sandy beach on Kauai (West End).
It appears to have some very small shells (scale=rock is 4x3x1 cm) of some sort embedded in what appears to me to be a calcium-based matrix (coral reef?), on top of lava. I can't tell if the shells are fossilized. The shells also appear to have bored into/through the lava.

Send thoughts and ideas to: Toby T.

Email: Toby

Ideas:

• just viewed your mytery stone. It is indeed covered with broken off vermetidae but the holes are most probably from boring bivalves. There is a wide diversity of boring bivalves. For us collector’s, the trouble is to get them out complete.
  Best regards, Guido P.

• The holes in the rock may have been cavities as the lava formed. If it spilled into, or erupted under the sea the water trapped by the lava forms steam which forms holes.
  Allen A.

• Magilia caperata looks similar to these also in terms of size. I am no expert in this group but believe it is a cosmoploitan species. I have never noticed it actually bore into rock before.
  Geoff M.

• Arca settle into a crevice but stay there and keep enlarging it.
  If the rock is really coralline algae, there are a number of bivalves that bore into it. I have found them diving at about 80 feet seeing them when a coral was broken. To photo them, I brought the surrounding coral back home.
  Aloha, Wesley T.

• The "fossils in rock" looks like vesicular basalt with calcareous algal overgrowth, with vermetid gastropods attached or etched-in. Andrew G.