English: Beryllium is a relatively inert, hard, medium grey metal, which is very light. It is nearly transparent for X-rays. Beryllium is not often used, as it is quite expensive and very toxic, in its elemental form as in many of its compounds. However, it is an important ingredient in many valuable gemstones, like beryl, aquamarine and emerald. Clear beryl was used for optical lenses in former times. The extremely unstable isotope beryllium 8, which has a half-life of 67 quintillionths of a second, plays an important role in the universe. Advanced stars fusion helium 4 to carbon 12 via this isotope