Optical activity that depends on the intensity of light has been predicted and then observed in lithium iodate crystals. In comparison to lithium iodate, extrinsic 3d chirality associated with oblique illumination of metasurfaces lacking two-fold rotational symmetry was found to lead to 30 million times stronger nonlinear optical activity in the optical part of the spectrum. At microwave frequencies, a 12 orders of magnitude stronger effect than in lithium iodate was observed for an intrinsically 3d-chiral structure.