Within the mineralogy the heavy liquid diiodine methane (CH212) is used in combination with the immersion method to determine the refractive index of gemstones. It is also usable for separation and density determination of unknown heavy minerals.
How to use diiodomethane and practical examples can be found in the articles: Measuring the refractive index with a refractometer or viewing inclusions with a stereo microscope (bright field microscopy).
You can find background information on examinations with stereomicroscopes or with immersion liquid in the Methods article Microscopy . You can find out more about measuring with refractometers under Refractometry .