The most basic difference in masks lies in the number of lenses. Single-lens masks were the first developed and remain a popular choice worldwide because they provide a broad, uninterrupted view. Two lens masks are typically lower in internal volume than single-lens masks and quicker to clear. Almost all modern mask skirts are made of silicone, with the primary differences being the quality and suppleness of the material, and its color. A majority of divers prefer masks with clear silicone skirts which let in the maximum amount of light. Those who tend to focus on a single object, (photographers, underwater surveyors, free divers) will choose a dark skirt to avoid the distracting glare from external light.
Crystal silicone - The highest grade silicone available for a soft comfortable seal