To adequately microscopically characterize opaque particles, and some finely divided, high refractive index, and transparent particles, one must use top light. This can be done with a built-in or accessory vertical illuminator. It can also be done without a vertical illuminator using any PLM with any separate microscope illuminator focusable to a small bright spot. Aimed downward onto the slide preparation at a steep angle, but missing the objective, and with the transillumination subdued or turned off, the color and surface texture of opaque particles can be readily discerned. The objective must have a long working distance, usually, therefore, a 10X or lower magnification.
Pseudo opaque particles (submicrometer particles of high index) are reaify transparent but hey may be colorless or any color. Top light brings out their true color - black, if opaque, but they may be red (Fe2O3), green (CR2O3), yellow (lead-tin yellow) or white (TiO2).