The pavilion of a gem refers to the portion below the girdle (its widest point). I often describe the crown (the portion of the gem above the girdle) as the lens of the gem and the pavilion as the mirrors. The pavilion on a round brilliant cut gem has 24 facets or 25 if there is a culet added.
The depth of the pavilion varies depending on the gem type being used. In theory, the higher the refractive index of a gem, the less steep the sides of the pavilion need to be in order for the light to bounce off it and be reflected to the pavilion facet opposite. That said, with most coloured gemstones the pavilion is cut shallower than its optimum; this is because when cutting the gem, there is often more focus placed on showing its best colours, rather than optimising its brilliance.
