What type of contact lens is known for being hard but can be worn for an extended period?

Gas permeable lenses are a type of contact lens known for their rigid structure, which allows for effective oxygen permeability while still providing a stable and comfortable fit. Unlike soft contact lenses that are made from flexible materials, gas permeable lenses maintain their shape and provide excellent vision correction, particularly for individuals with astigmatism or other refractive errors.

One of the significant benefits of gas permeable lenses is their ability to be worn for extended periods, ranging from daily wear to extended wear options that can be used overnight for several days. This is due to their ability to allow oxygen to pass through to the cornea, which is essential for maintaining eye health, especially during prolonged use.

In contrast, hydrogel lenses, while also designed for comfort, are made from a softer material that does not typically provide the same level of vision correction or oxygen permeability for extended wear. Soft contact lenses are generally easier to adjust to but may not offer the durability and longevity of gas permeable lenses. Daily disposable lenses are meant to be used for a single day before being discarded, making them unsuitable for extended wear usage.

By understanding these distinctions, it's clear why gas permeable lenses hold the characteristics that match the description in the question as both hard and suitable for extended wear