It takes up to 60 hours to make, but is less than a millimetre wide. It is as hard as a diamond, but casually mislaid a million times a day. The ball in the ballpoint pen, or Biro, is 70 years old today.

Sales of the ballpoint pen are stronger than ever, says the leading manufacturer, Bic. Last year it sold 216 million in Britain, a 2 per cent increase on the previous year. Worldwide, an estimated 15 million are sold every day. Predictions that the writing was on the wall for pens in the digital age seem as premature as the dream of the paperless office.

Since 1950, Bic has sold more than 100 billion ballpoint pens globally - enough to draw a line to the moon and back more than 320,000 times. Yet few objects are as easily lost, stolen or chewed in lieu of cigarettes. Douglas Adams, author of The Hitchhiker's Guide to the Galaxy, suggested that unattended ballpoints independently made their way 'through wormholes in space' to 'a planet entirely given over to Biro life forms'.

The father of one of the 20th century's most successful inventions was Laszlo Biro, a Hungarian who dabbled in medicine, hypnosis, the oil industry, motor racing and painting. It was as a journalist that he became frustrated by the scratchy nibs of fountain pens which splattered ink and tore paper. One day, he was in a Budapest printing shop and saw an ink that dried as soon as it touched paper. 'It got me thinking how this process could be simplified right down to the level of an ordinary pen,' he recalled later.

The quick-drying ink used in printing was too thick to flow from a fountain pen, so Biro and his brother, George, a chemist, came up with a new design. After several years of experiments, they replaced the pen's metal writing nib with a tiny ball bearing in its tip. As the pen moved along the paper, the ball rolled, picking up ink from the ink cartridge and depositing it evenly on the paper. Whereas earlier ballpoints had relied on gravity to make the ink flow, the Biro used a pressurised tube and a capillary action to avoid leaking or blotting.

Biro took out a British patent on 15 June, 1938. But he and George were forced to flee from Hungary during the Second World War and settled in Argentina, where they began small-scale production in 1944, pricing the prototypes at £27 each under the name Eterpen. Their first bulk order was from the RAF, which needed more than 30,000 Biros so that navigators could write at high altitude, where fountain pens tended to leak.

Biro sold his patent to Baron Marcel Bich of France for $2m, around £11.6m in today's money. In 1950 Bich (who later dropped the 'h' to become Bic) launched his own cheap, disposable, mass-produced ballpoint pen. It conquered the market and in 1965 the French government approved its use in schools, with other countries following suit.

Biro's daughter, Mariana, 73, said recently: 'The Biro was my father's greatest invention. I'm so proud that the name lives on. He used to hear people say the ballpoint was ruining writing skills. He would smile and say, "Well, writing comes from the heart. If we can help the hand to perform the task, what is so wrong with that?"'

The design has evolved. The ball is now made from a tungsten carbide pellet, which is ground between plates using a grinding paste to form the spherical shape, a process that can last up to 60 hours. The finished ball is less than 1mm in diameter and as hard as diamond, ensuring that it will not pit or develop 'flat' areas on its surface during use. The barrel was made hexagonal for a better grip and has a small hole to allow air into the capillary tube so the rotating ball creates only a partial vacuum, ensuring the ink flows properly.

Bic's Cristal ballpoint pen can cost 10p-15p and produces around two kilometres of writing. The firm has 24 factories which it says in 2006 produced direct greenhouse gas emissions equivalent to 13,000 tonnes of CO2. The biggest environmental cost of a ballpoint pen is the oil used to make five grams of plastic for each pen. But Bic says the fuel consumed by a low-powered car over 100km requires enough oil to make 3,200 pens. It also says that because Bics 'are not toxic and contain few materials, the environmental impact at the end of their life is minimal'.

Laszlo Biro worked on more than 100 inventions in all, and pursued a career as a Surrealist painter. He died in 1985 at home in Buenos Aires and every year Argentinian Inventors Day is celebrated on his birthday.