Found embedded in sea pork wash-ashore. Paper thin, about an inch long. Heavy nacre on the interior. Exterior was covered by the sea pork.
Lucky shot of it using a huge jet of water from its siphon to shoot completely out of the water (and the dish)! It had been sulking in the dish after being captured by a group of students; see rock-coloration in second photo. The third photo shows it going back to sulking after its unsuccessful escape from the paparazzi.