Modern ooids from Joulters Cay, when baked at 500 °C, turn various shades of black depending upon the organic content. Mucus‐rich laminae occur at quasi‐regular intervals of a few micrometres within the cortex. When mucus is still present, it turns black; when it is absent, there is a gap. The cortex consists of two types of aragonite: (1) ‘batons’ of circular cross‐section capped by a single 0·1‐μm (100‐nm) ball, which can be interpreted as a single nannobacteria cell that precipitated the baton; (2) elongate crystals made of multiple rows of minute balls of about 0·03 μm (30 nm), which may or may not have been small organisms in the size range of viruses. There are also some crystals that show no evidence of organic precipitation. Hardground cementation begins with the formation of a terminal mucus‐rich ring on the ooid that bakes black and is crowded with 0·1‐μm (100‐nm) balls. Some ooids are then joined by a meniscus also made of mucus with aragonite crystals. The final, most abundant hardground cement forms a fur of inorganic aragonite crystals often shaped like plywood sheets, although some ‘organic’, elongate crystals composed of ≈0·03 μm (≈30 nm) balls are also found in the later cement. For a century, ooids have been known to be closely associated with organic matter; this paper goes further and proposes that the bulk of the ooid may be precipitated by nannobacterial processes. Hardground formation, in the beginning, may also be a microbiological process, but most cementation is accomplished inorganically.

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