A rock made of small pearls


The spheres that you see in this photograph are composed of calcium carbonate and have a diameter of approximately 1mm.


They are called ooids and the rock that contains them is known as oolite. They are incredible objects which to this day form in crystalline tropical seas.


Ooid sands are white and so the sea where they form takes on a clear turquoise hue, such as in some parts of the Bahamas.


Each ooid develops around a single grain, like a pearl, and is made up of aragonite (a form of calcium carbonate), like pearls.


Unlike pearls, they are not part of a shell. Ooids form in environments where the current and the waves keep the sea in constant motion. There are always suspended grains of sand present which become encrusted with aragonite. The encrustation proceeds evenly in all directions if the particle never comes to rest on the seabed, in which case the aragonite would accumulate only on one side, and when the ooid becomes too heavy to remain suspended, the particle falls to the bottom.

Name: Oolite
Classification: Chemical sedimentary rock, Oolitic grainstone
Mineral composition: Calcite (from aragonite alteration), other
Fossils: foraminifera (in the bottom centre ooid)
Location: Giau Pass, Mount Cernera(46°28’22.0″N12°04’30.1″E)
Formation: San Cassiano formation
Erà: Lower Carnian (approximately 235 million years ago)
Depositional environment: Carbonate substructure, dry oolitic