In situ measurement of stress in polycrystalline samples forms the basis for studies of the mechanical properties of materials with very broad earth science and materials science applications. Synchrotron x rays have been used to define the local elastic strain in these samples, which in turn define stress. Experimental work to date has been carried out on a prototype detection system that provided a strain measurement precision >10(-4), which corresponds to a stress resolution >50 MPa for silicate minerals. Here we report operation of a new, permanent, energy dispersive detection system for white radiation, which has been developed at the National Synchrotron Light Source. The new system provides differential strain measurements with a precision of 3 x 10(-5) for volumes that are 50 x 50 x 500 microm(3). This gives a stress precision of about 10 MPa for silicate minerals.