Beryllium

Beryllium is a light, gray metal that is, for its weight, much stiffer than steel. Its main ore is the mineral bertrandite, and it also occurs in beryl, whose green and blue gem forms are emerald and aquamarine. Most of the world's supply is mined in Utah.

X-rays pass through it easily, so it is used as the window in X-ray tubes and detectors. The mirrors of the James Webb Space Telescope are made of gold-coated beryllium. A little beryllium mixed into copper makes a strong, springy alloy for electrical contacts and spark-free tools. Its dust is toxic to breathe, so it is handled with great care.