The light passing through the stone allows examination of the interior of the gem, while the exterior is seen best using the light source above the microscope stage. Dark-field illumination is the standard for examining gems for inclusions and fractures. The gem is illuminated with a cone of light passing through the stone rather than directly reaching the objective lens. The gem appears bright with a dark background. Bright-field illumination, or transmitted light, illuminates the stone directly from below and the iris diaphragm controls the opening or the lessens the glare.
Inclusions look dark against the bright background.
Oblique reflected or horizontal light requires the high-intensity fiber optic illuminator to light the stone at an oblique angle or horizontally. Near-vertical overhead lighting is used to detect external features, such as abrasion on facet junctions, scratches, pits, nicks, or polishing marks. The illumination should strike the gem at right angles to maximize the reflection and minimize refraction. Types of Distinguishing Characteristics Observing surface abrasion provides information on physical properties, and heat treatment and enhancements, while internal features or inclusions determine the clarity grade and value of the gem. These clarity features are found with magnification, and are termed blemishes when they are external and inclusions when internal.