Rheinberg illuminiation is an old technique based on darkfield illumination. The difference is that the outer annulus is colored by one or more filters, which provides coloration to the specimen, possibly dependent on the angle of incidence. Additionally, instead of a black background provided by an opaque central stop, the central stop is often a dark, but not black color, which determines the background of your image. Selection of the central stop and annulus colors determine how the image will look, so many possible effects can come out of this one technique. Transparent crystals, such as salt and snowflakes, are often imaged with this contrast enhancement technique.
Not many microscope manufacturers provide the appropriate filters for this, but they can be purchased from a third-party, or made by you!
The images below were made with a 4x Plan objective on the Diaphot TMD with simple hand-made filters cut from shim stock. It is sodium chloride crystals (table salt) in transmission.
Along the same lines, I wanted to see what sodium hydroxide (NaOH) crystals looked like. Unfortunatly, I took too long (about a minute), before they started soaking up moisture from the air (76.6°F, 50% RH). The sequence below is over a span of 5-10 minutes!
for navigation!