From the CRC handbook of Chemistry and Physics, CRC Press, Cleveland, 55th ed., 1974-75 "The Elements" by C.R. Hammond. page B-8
Boron (Ar. Buraq, Pers. Burah), B; at. wt. 10.81; at. no. 5; m.p. 2300 C; b.p. sublimes 2550 C; sp. gr. of crystals 2.34, of amorphous variety 2.37; valence 3. Boron compounds have been known for thousands of years, but the element was not discovered until 1808 by Sir Humphry Davy and by Gay-Lussac and Thenard. The element is not found free in nature, but occurs as orthoboric acid usually in certain volcanic spring waters and as borates in borax and colemanite. Ulexite, another boron mineral, is interesting as it is natures's own version of "fiber optics." By far the most important source of boron is the mineral rasorite, also known as kernite, found in the Mojave desert of California. Extensive borax deposits are also found in Turkey. Boron exists naturally as 19.78% (5)B(10) isotope and 80.22% (5)B(11) isotope. High-purity crystalline boron may be prepared by the vapor phase reduction of boron tricholoride or tribromide with hydrogen on electrically heated filaments. The impure, or amorphous boron, a brownish-black powder, can be obtained by heating the trioxide with magnesium powder. Boron of 99.9999% purity has been produced and is available commercially. Elemental boron has an energy band gap of 1.50 to 1.56 electron volts, which is higher than that of either silicon or germanium. It has interesting optical characteristics, transmitting portions of the infrared, and is a poor conductor of electricity at room temperature, but a good conductor at high temperature. Amorphous boron is used in pyrotechnic flares to provide a distinctive green color, and in rockets as an ignitor. The most important compunds of boron are boric, or boracic acid widely used as a mild antiseptic, and borax (Na2B4O7 10H2O), which serves as a cleansing flux in welding and as a water softener in washing powders. Boron compunds are used in production of enamels for covering steel of refrigerators, washing machines, and like products. Boron compounds are also extensively used in the maufacture of borosilicate glasses. The isotope of Boron 10 is used as a control for nuclear reactors, as a shield for nuclear radiation, and in instruments used for detecting neutrons. Boron nitride has remarkable properties and can be used to make a material as hard as diamond. The nitride also bhaves like an electrical insulator but conducts heat like a metal. It also has lubricating properties similar to graphite. The hydrides are easily oxidized with considerable energy liberation, and are being studied for use as rocket fuels. Demand is increasing for boron filaments, a high strength, lighweight material chiefly employed for advanced aerospace structures. Amorphous boron (90%-92% grade) costs about $12 to $30/lb. depending on the quantity ordered. Elemental boron is not considered to be a poison, but assimilation of its compounds has a cumulative poisonous effect.
