Coquina

The pretty little coquinas (genus Donax, family Donacidae), often called butterfly shells or wedge shells, are distributed throughout the world. Perhaps the most noted species is the small Donax variabilis Say, so commonly found on the beaches of Florida. Coquinas live in droves just beneath the surface of the sand. Collecting coquinas is both a fascinating and tantalizing experience.

Source: wiktionary

They were filled with "coquinas," tiny butterfly-shaped clam shells that gleamed with all the colors of the rainbow. Mario poured the sand and its brilliant treasure into my hands. I stared at the beauty and thought about God, Sabbath, and friends. A few weeks earlier this day had been an ugly thing to me, like the oily sand beneath my feet. Now, like the colorful coquinas, Sabbath had become God's beautiful gift to me and my growing family.

Source: wiktionary

The isthmus is nowhere more than about 8 or 9 feet above the water surface. The top soil is generally sand, with underlying coquina rock—a conglomerate of small broken shells held together by some natural cement.

Source: wiktionary

[“The Town”, page 22] In former times, most of the houses were of coquina (a loose shell-stone, quarried on the island opposite [Anastasia Island]), but this material is now almost entirely superseded by wood. Roomy and moder wooden houses may not so picturesque as the weather-stained coquina dwellings they have supplanted, but they are immeasurably more comfortable to live in. […] [“Fort Marion”, page 50] The Fort [Fort Marion, now Castillo de San Marcos] is built of coquina, which in its day was considered a very excellent material for this purpose, since cannon balls would sink into the wall without shattering it as they would harder stone.

Source: wiktionary