The thermal conductivities of fused quartz and of crystalline quartz (heat-flow at 90° to the optic axis) have been measured by steady-state plate methods; the former over the range -150 to 50° C, and the latter at 28° C. The most probable thermal conductivity values of these two materials have been formulated as functions of temperature, having in mind their use as calibration specimens for apparatus measuring the thermal conductivities of rocks, important in the calculation of terrestrial heat-flow. Detailed description is given of apparatus and measurement methods, and the results of other observers are listed and briefly discussed.