The keyhole design is a classic shape, named after the keyhole-shaped gap between the bridge and nosepads. While most eyewear frames are made of plastic and have a seam all around the edges from the injection mold, our sunglasses are made from blocks of cellulose acetate or horn that are carved into their final shape. This process gives the frames greater flexibility, durability, and depth of color. In addition to the acetate frames, we also offer horn frames, in dark color horn and light color horn. Horns are ethically sourced from water buffaloes who've reached the end of their working age and are never killed solely for their horn. As every horn varies in color, each piece has a unique appearance. Spring hinges allow the arms to flex and conform to your head, for greater comfort. Mazzucchelli CR-39 polarized lenses. 100% UV 400 protection. Meets US ANSI Z80.3-2009, EU EN 1836:2005 standards. Frames suitable for prescription lenses.