Cellulose acetate is a plant-based plastic supplied as sheet by makers such as Mazzucchelli in Italy. Frames are cut, tumbled and hand-polished from it, which is why acetate frames carry deep colour and a glassy finish and why they cost more than injected plastic. Bio-acetate uses plant-based plasticisers.
Metal frames are made from monel (a nickel-copper alloy), stainless steel, nickel silver or beryllium copper, cut and soldered, then plated. Titanium and beta-titanium are the premium: half the weight, no corrosion, hypoallergenic. Japan's Sabae district pioneered titanium eyewear in the 1980s; China now makes most of the volume.
TR90 (Grilamid, a nylon) and Ultem are injection moulded. Once a mould exists, frames are cheap and consistent, light and flexible, which is why TR90 dominates children's, sports and budget frames. The mould is the cost: a new design needs tooling that a factory will only make against a minimum order.
Combination frames mix acetate fronts with metal temples, or metal fronts with acetate tips. Wood, horn, carbon fibre and 3D-printed polyamide and titanium are niche premium materials.