Enantiomers, or stereoisomers, have crystal structures that are mirror images of each other and are thus handed, like our right and left hands. Low-quartz（SiO₂）and Berlinite（AlPO₄）have enantiomers belonging to a space-group pair, P3₁21（right-handed screw）and P3₂21（left-handed screw）. We show that resonant Bragg diffraction using positive and negative circularly polarized X-rays at the resonant energies reveals the handedness of crystals by X-ray helicity coupling to a crystal screw axis. Our results are of general importance and demonstrate a new method to directly study chiral motifs in structures that include biomaterials, liquid crystals, magnets, multiferroics, etc.