Dark Field Illumination
An additional hindrance is the absence of (microscope) dark-field illumination. With loose diamonds and a gem-scope, locating inclusions is a snap. Dark-field illumination makes the defects stick out like a sore thumb. With a mounted diamond, the mounting's background usually does not allow the scope's dark- field to do its its part. As a result, spotting inclusions is much more difficult in mounted stones.