The Bic Cristal is the best selling pen in the world; the 100 billionth was sold in September 2006.
The Bic Cristal's industrial design has been acknowledged by the Museum of Modern Art in New York City as part of the museum's permanent collection. Its hexagonal shape was taken from the wooden pencil and yields an economical use of plastic along with strength and three grip points giving high writing stability. The pen's transparent polystyrene barrel shows the ink-level. A tiny hole drilled in the barrel's body keeps the same air pressure both inside and outside the pen. The thick ink flows down due to gravity from a tube inside the barrel to feed a ball bearing which spins freely within a brass/nickel silver tip. In 1961 the stainless steel ball was replaced with much harder tungsten carbide which is vitrified by heat, then ground down and milled to an accuracy 0.1 micrometre between spinning plates coated with industrial diamond abrasives. Since 1991 the pen's streamlined polypropylene cap has had a small hole to reduce the risk of suffocation if the cap is inhaled.