What metal is JWST's primary mirror primarily made of?

JWST’s primary mirror is primarily made of beryllium.

The mirror is built from 18 hexagonal beryllium segments, which fit together into a 6.5-meter-wide primary mirror. Each segment is coated with a very thin layer of gold, but gold is the reflective coating—not the main structural material.

Beryllium was selected because it is lightweight, stiff, and behaves predictably at the extremely low temperatures required for infrared astronomy. Its low density helped engineers reduce the mirror’s mass, while its thermal properties help the segments retain their carefully shaped surfaces in space.

A common mix-up is to answer “gold,” since JWST’s mirror visibly has a golden appearance. The gold coating improves reflection of infrared light, whereas the beryllium provides the mirror’s underlying structure. The completed mirror has a light-collecting area of about 25 square meters, roughly six times Hubble’s.