Oolite

Oolite is a sedimentary rock — almost always a limestone — composed of ooids: tiny, sand-sized spherical grains built of concentric layers of calcium carbonate around a nucleus. The name comes from Greek for "egg stone," because the grains look like fish roe.

Each ooid forms by the accretion of carbonate around a seed particle (a shell fragment or sand grain) as it is rolled back and forth by waves and currents. When these grains are cemented together, they form oolitic limestone.

The rock typically has a granular, sugary texture and a uniform cream-to-tan color. Larger versions of these grains (over ~2 mm) form a related rock called pisolite.

Ooids form in warm, shallow, agitated marine waters that are supersaturated in calcium carbonate — classic settings include the Bahama Banks, the Persian Gulf, and ancient shallow carbonate platforms.

As tiny nuclei are rolled by wave and tidal currents on shallow sandbars and shoals, thin layers of carbonate (calcite or aragonite) precipitate around them, building concentric coatings like a pearl. Continual rolling keeps the grains rounded and evenly coated.

The ooid sand is then buried and cemented into oolitic limestone. Some oolites form from iron minerals (ironstone ooids) rather than carbonate, but carbonate oolite is by far the most common.

Look for a limestone made of tiny, uniform, rounded grains resembling fish eggs or tiny pearls — a hand lens reveals the spherical ooids, sometimes with visible concentric layering.

As a carbonate it fizzes in dilute acid. It is moderately soft (Mohs ~3-4) and has a granular, sugary feel.

Look-alikes: Regular limestone (lacks the distinct rounded ooids), sandstone (grains are quartz and not concentrically layered), and pisolite (same structure but larger, pea-sized grains). The roe-like spherical grains are the diagnostic feature.

Oolitic limestone is an excellent and historically important building stone — easy to cut yet durable. England's famous Bath Stone and Portland Stone are oolites used in countless monuments, including St. Paul's Cathedral, and Indiana Limestone in the US is similar.

It is also used as crushed aggregate, agricultural lime, and a cement raw material. Many oolitic limestones are good petroleum reservoirs because the spaces between ooids hold oil and gas.

Oolite is not used in jewelry, but its uniform texture and workability make it a prized architectural stone, and it is geologically valuable as a record of warm, shallow ancient seas.