High quality wine glasses are often made of lead crystal. Other than aesthetics, one factor is lead crystal is generally considered to have a higher index of refraction, thus changing the angle of the light passing through the material. Lead crystal is also rougher than glass on a microscopic level, allowing wine in the glass to breathe more efficiently when swirled in the bowl. They are also heavier. Using lead in the crystal matrix also offers several advantages in the material's workability during production. Wine glasses are generally not coloured or frosted as this would impede the appreciation of its colour. An exception to this rule is the hock glass.