A nonmetallic element that is amorphous and brown or crystalline and black, and is extracted chiefly from kernite and borax and used in flares, propellant mixtures, nuclear reactor control elements, abrasives, and hard metallic alloys. Atomic number 5; atomic weight 10.811; melting point 2,075°C; boiling point 4000°C; specific gravity (crystal) 2.34, (amorphous) 2.37; valence 3. See Periodic Table.