Sometimes caused by the lack of inclusions!
Have you ever looked at a gem such as a Mozambique Garnet and the outer edge looks darker than the middle of the table? This is due to less light being reflected from this area of the gem. The effect varies from gem to gem, but the biggest factor is the angle of the pavilion facet: the steeper the angle, the greater the extinction.
Heavily included gems tend to display less extinction. The inclusions scatter light in different directions within the gem, often returning brilliance to the outline of the gem.
Measuring extinction is a valuable tool in helping gemmologists recognise origins for certain gemstones. For example, a Burmese Ruby normally shows less extinction than a Thai Ruby due to its higher level of fluorescence.
Often gem dealers will incorrectly claim that a gem has varying tones or that it is dichroic, rather than realising that it is caused by extinction. It’s understandable though: I have only ever seen the subject in two gemstone books and I have read hundreds! I recently read a trade article by Richard Hughes, who has written extensively on Rubies and Sapphires, where he also described its occurrence as the “black out” area of a gem.
