An organic nonlinear optical crystal having soft and brittle in nature has been machined by an ultra-precision turning lathe with a sharp diamond tool under various cutting conditions in order to obtain optical surfaces. The finished surfaces can be classified into three categories, that is, fracture mode, ductile mode and ductile & fracture mode, respectively. These surface categories ace remarkably affected with the crystal orientation and rake angle of cutting tool. The turned surface roughness is affected by the crystal orientation, rake angle and feed rate, but not affected by the cutting speed and depth of cut. The optical surface of 1.57nm rms or 12.3nm Rmax in surface roughness and 80nm in flatness has been obtained by using a diamond tool having a rake angle of -20 degree. A 532nm green laser of 1.4mW in output power has been generated by using the diamond-turned chalcone in an intra-cavity SHG device.