What is light leakage in a gemstone?
By Jared James · Last updated 21 May 2026
Definition
Light leakage in a faceted gemstone describes what happens when light entering the crown passes straight out through the pavilion rather than being reflected back up to the viewer's eye. The result is a dull, dark or washed-out patch in the stone, sometimes called a window, where the face of the stone simply shows the surface it is resting on rather than sparkling brilliance. Leakage is most often caused by shallow pavilion angles that fail to reflect light internally, making cut quality the key factor in avoiding it.
Frequently asked questions
- What causes light leakage in a diamond or gemstone?
- The main cause is a pavilion cut at an angle too shallow to achieve total internal reflection. When the pavilion facets do not redirect light back through the crown, that light escapes through the bottom and the stone looks darker or glassy in those areas.
- How can you check for light leakage in a stone?
- Hold the stone face-up over a piece of white paper or a dark cloth. If you can see through sections of the stone as if it were a window, or if large areas look dark while light hits the edges, that suggests significant leakage.
- Is light leakage the same as extinction in a gemstone?
- They are related but different. Light leakage means light escapes through the base rather than returning to the eye. Extinction refers to dark areas that appear because light is not being reflected back from certain facets, which can happen even in a well-cut stone depending on the viewing angle.
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