Captain Nemo's pearl

The shells were a little open; the captain came near, and put his dagger between to prevent them from closing; then with his hand he raised the membrane with its fringed edges, which formed a cloak for the creature. There, between the folded plaits, I saw a loose pearl, whose size equaled that of a coconut. Its globular shape, perfect clearness, and admirable luster made it altogether a jewel of inestimable value. Carried away by my curiosity, I stretched out my hand to seize it, weigh it, and touch it; but the captain stopped me, made a sign of refusal, and quickly withdrew his dagger, and the two shells closed suddenly. I then understood Captain Nemo's intention. In leaving this pearl hidden in the mantle of the tridacne, he was allowing it to grow slowly. Each year the secretions of the mollusk would add new concentric circles.

-- Jules Verne Twenty Thousand Leagues Under the Sea