French, 13th century. Tart mould ring. Photo courtesy Sotheby's

Gold, set with a point-cut garnet - bezel: 14mm., 9/16 in. - ring size Q. Estimation 2,500 — 3,500 GBP 

The tart mould ring is named for the shape of the underside of the bezel. The form is meant to mimic the natural crystalline shape of the gems it holds. A similar ring is illustrated in A. B. Chadour, Rings. The Alice and Louis Koch collection, Leeds, 1994, vol. 1, p. 170, fig. 560.