Ooids on the surface of an oolite limestone. Carmel Formation (Middle Jurassic
) of southern Utah
Thin-section of calcitic ooids from an oolite
An ooid is a small rounded sedimentary rock. Like sand grains, ooids have a diameter of 2 mm or less. Larger grains formed the same way as ooids are called pisoids. Rock made of ooids is oolite.