oncolite (on'-co-lite). A small, variously shaped, concentrically laminated, calcareous sedimentary structure, resembling an oolith, and formed by the accretion of successive layered masses of gelatinous sheaths of blue-green algae. It is smaller than a stromatolite and generally does not exceed 10 cm in diameter. Also spelled: onkolite. Cf: catagraphite.