The specific surface free energy of a ruby single crystal was experimentally obtained using the contact angle of liquid droplets on the crystal surfaces, and then compared with the morphology of each crystal. The obtained specific surface free energy satisfied the Wulff's relationship, even though the synthesized crystals should be in growth form. We propose an idea of steric energy density, which gives another interpretation for the Wulff's relationship. (C) 2007 Elsevier B.V. All rights reserved.