Beryllium, periodic table Be, is a light metal with 4 neutrons. Beryllium is relatively rare on Earth and the same should be true for Mars.
Beryllium is used as an element in light alloys, as a heat transfer material and as a light metal. It has nuclear properties that make it useful as a x-ray window. Beryllium and many of its compounds are toxic and require careful handling.
Beryllium should be found on Mars in the form of beryl, Be3Al2Si6O18, that is the geological mineral mined on Earth. Emeralds are a variety of Beryl. Beryl can be broken down into Beryllium oxide (BeO) or reacted through a complex fabrication process into beryllium chloride that is separated by electrolysis to obtain the metal.