Gold particles embedded in a polymer are studied using high resolution electron microscopy, nanodiffraction, and image processing. These particles have interesting properties such as nonlinear optical susceptibility. It is found that most of the nanoparticles are single crystals, and twins and planar defects are scarcely observed. On the other hand, it is found that nanoparticles do not show smooth facets but they are instead very rough. The effects of this extended roughness in the optical properties are discussed. ©1996 American Institute of Physics.