A new derivative of Arizona sand, ISO Medium Test Dust, is coming into use as a liquid optical particle count calibrant. ISO Medium Test Dust and optical particle counters are used to test filtering efficiency. Particle shape plays a role in both filtering and optical detection. To establish a new Standard Reference Material for the fluid power industry, NIST performed scanning electron microscopy on large numbers of medium test dust particles over the size range of 0.5 um to 50 um. Aspect ratio, longest chord to projected area diameter ratio, and circularity have been derived from the boundaries of the SEM images.