An oval cut diamond, comprising a girdle having a border line formed in an elliptic shape or a shape similar to the elliptic shape, a crown at its tip part having, at the upper part of the girdle, a table facet formed in an octagonal shape, and a pavilion at the lower part of the girdle. The so-called deformed oval brilliant cut diamond is formed by rotating either of the crown and the pavilion of a normal brilliant cut diamond by approximately 1/16 turn around its center axis. Where the radius of the girdle in the major axis direction is (a) and the radius of the girdle in the minor axis direction is (b), the ratio (b/a) of the radius in the minor axis direction to the radius in the major axis direction is 0.6 or larger. A pair of pavilion main faucets positioned oppositely to each other with respect to the center axis comprise a pair of crown main faucets or a pair of star faucets opposed to each other on both sides of the girdle. Since these two pavilion main faucets, two crown main faucets or star faucets, and the table faucet comprise a common vertical plane in these faucets, the brightness of reflected light coming to the upper side of the table faucet and the crown faucet is intensified.