We describe experiments characterizing a new nonlinear optical crystal, YCa/sub 4/O(BO/sub 3/)/sub 3/ (YCOB). This crystal has a number of advantages over other commonly available nonlinear optical crystals. It has a higher nonlinear coefficient than KDP, can be fabricated to large sizes (/spl sim/3-in diameter, 8-in length), and has a high damage threshold. Moreover, this new nonlinear optical crystal is nonhygroscopic, has good optical quality and mechanical properties, allowing easy optical polishing. This crystal, YCa/sub 4/O(BO/sub 3/)/sub 3/, commonly termed YCOB, is one of a family of new nonlinear crystals, the oxyborates, that include RECa/sub 4/O(BO/sub 3/)/sub 3/ (RE=La/sup 3+/, La/sup 3+/, Y/sup 3+/, Sm/sup 3+/, Gd/sup 3+/, Er/sup 3+/, and Nd/sup 3+/). In this paper, we also successfully demonstrate a technique for improving the nonlinear optical properties of this crystal. This technique, ion substitution, has previously had limited success with other crystal hosts. However, the inclusion of yttrium in YCOB provides the opportunity to exploit this technique. Yb/sup 3+/, which has larger mass, but approximately the same atomic size as Y/sup 3+/ can be substituted into the crystal structure without introducing stress and nonuniformities. A systematic investigation of the linear and nonlinear characteristics of several crystals doped with various levels of Yb demonstrate that selective substitution of Yb in YCa/sub 4/O(BO/sub 3/)/sub 3/ improves the second-harmonic conversion efficiency by increasing the optical nonlinearity.