Anomalies of optical rotation, light depolarization, birefringence and dilatation of crystal quartz under σyy uniaxial stresses confirm the existence of an incommensurate (1 q ) stripe phase, recently expected theoretically and discovered by neutron scattering, between the β and the INC (3 q) phases. The phase diagram is determined to a better resolution than with neutron diffraction. These anomalies allow us to obtain also the phase diagram of the 1 q phase under σ xx stresses. Both phase diagrams show the presence of the 1 q phase over a small temperature range (a few hundreth K), even at zero stress. As expected from theory, the variation of its temperature interval is found to be larger with σ yy stress than with σxx stress and the 1 q ↔ 3 q transition appears to be first-order.