Abstract: The present invention discloses a round brilliant cut diamond. The diamond includes a crown, a girdle, and a pavilion, the cut crown includes one regular octagon table, eight first facets, eight second facets, and sixteen third facets, wherein the cut pavilion has sixteen quadrangular fourth facets, sixteen quadrangular fifth facets, and sixteen sub-triangular sixth facets. The sixteen fourth facets meet at a point, which forms an apex, and each fourth facet, fifth facet and sixth facet meet a point. A depth of the fourth facets is smaller than 21.6% of the total girdle diameter D. The present invention also discloses methods for preparing above mentioned round brilliant-cut diamond.