Cave pearls, up to 8 cm long, collected from dried-out rimstone pools in Old Man Village cave on Grand Cayman, British West Indies, have nuclei (e.g., bone) that are encased by laminated cortices. These laminae are composed of trigonal prismatic calcite crystals, calcite crystal bushes, or particulate calcite that is formed of filamentous microbes (four morphotypes), spores (seven morphotypes), extracellular polymers (EPS), disorganized and organized calcite fiber crystals, and detrital calcite grains. Syntaxial overgrowths modified many of the fiber crystals and the trigonal calcite crystals. Thin, laterally discontinuous growth lines are formed of micrite that may have been precipitated in a microbial mat. Growth of the cave pearls involved abiogenic and biogenic processes. The trigonal calcite crystals and crystal bushes probably formed abiogenically whereas the particulate calcite laminae appear to have formed as microbes trapped and bound detrital grains to the surfaces of the cave pearls and indirectly mediated precipitation of the fiber crystals.
Water flow into the rimstone pools, which was linked to local rainfall, controlled the influx of detrital grains, water chemistry, and microbe growth into the pools and hence, the growth of the cave pearls. Periods of low water influx led to stagnation of pool waters that seemed to trigger periods of rapid abiogenic calcite precipitation and curtail microbial colonization. High flow rates slowed abiogenic calcite precipitation, promoted microbe growth, and fed detrital materials into the pools. Despite the lightless environment of the cave, the evidence clearly shows that the microbes and their associated EPS played a critical role in the growth and development of the cave pearls.