Scanning Electron Microscope
SEM is highly sophisticated equipment and only a few gemmological laboratories in the world have access to it. This technique is surface oriented and provides a very high magnification of the surface topography. With additional attachments, elemental analysis can be obtained.
- The basic features include a fine stream of electrons which are focused at a particular angle on the specimen which is scanned in a series of parallel lines.
- In this technique, secondary electrons are emitted from the specimen due to collision with the high energy incident beam.
- High energy backscattered electrons are emitted. These are electrons from the incident rays which have interacted with the specimen and then reflected out.
- The intensity of both emissions is collected, and converted with compatible software to a visible image.
- Polished specimens give a better result, since in rough specimens the variations are due to surface rather than the actual structure.
- In most cases the samples have to be coated with a layer of silver or gold for more accurate results.
- This is useful in the identification of gemstones, their origin and treatments such as the glass fillings in ruby (since ruby fluorescence masks these peaks in all the other mentioned techniques).