Ooid is one of the coated grains, is usually composed of calcium carbonate. It is commonly from shallow tropical seas, for example GBB. They can be cementde together to form carbonate rocks, named oolite.
Whether the primary mineral is of ooid is aragonite or calcite is determined by the Mg/Ca of the seawater. And there is a long secular variation in seawater, called aragonite sea and calcite sea.
In Lower Triassic of South China there are many Oolitic banks, which now are good oil and gas reservoir.
The picture is from my good friend, Feinan Li, who is interested in oolits.