There are cameras scanning the lobby. Cameras in the elevators. Cameras mounted on the ceiling of the visitors' lounge. And dozens of cameras scan each of the seven floors where 107 sorters hunch over glittering heaps of one of the most valuable commodities on earth: Diamonds.

Harry Oppenheimer House is the main sorting facility for De Beers Consolidated Mining Inc., the world's largest diamond producer. Eight of the 23 operating diamond mines in existence are in South Africa, and nearly every stone mined from them is sorted and graded here before leaving the country. And with something as tiny and as precious as diamonds - a gem-quality stone is worth thousands of times more per ounce than gold - the security measures go far beyond cameras.

Sorting diamonds is not a line of work for someone made nervous by close supervision.

The women sorters dress conservatively and plainly - no lace allowed. The men wear maroon sweaters and gray flannel trousers. The trousers have no cuffs, and the pockets of both men's and women's clothes are stitched up so that diamonds can't stray into the garments. A sign at the exit of each sorting area reminds workers: ``Check your clothing."

On a rare recent tour of the facility, De Beers officials instructed a visitor to suppress the urge to touch the piles of gems, which sparkle beguilingly even in their rough state. A security guard hovered closely to enforce the hands-off policy.

Not that it would be easy to get a rough diamond out of Harry Oppenheimer House, named for the former De Beers chairman and patriarch of the family that controls the worldwide diamond cartel.

The 14-story building is deliberately labyrinthine to frustrate thieves. It takes two elevators to get to the third floor. Electronic identity cards issued to employees give them access only to their own work areas.

About 10 million carats of diamonds go through Harry Oppenheimer House each year - about two tons of stones (a carat is 200 milligrams). De Beers knows to the thousandth of a carat what goes in and what goes out: Sorters can't go home for the day if the weight of their diamonds is off.

Sorters used to work with scales, but De Beers discovered that some light-fingered employees were substituting poor-quality stones for precious gems of equal weight. Now only supervisors have access to scales, and they are separated by glass walls from the sorters, though they are not beyond the gaze of the security cameras.

If any questions about honesty arise, De Beers has a full-body X-ray machine on the first floor; diamonds fluoresce when irradiated, even in body cavities. De Beers also has designated a room for strip searches, said Angus Galloway, the general manager of the sorting facility.

Sorters get extensive training in identifying the proper color and clarity of stones. They also get instruction in personal finance. The company believes that employees who stay out of financial trouble are less likely to steal.

The security measures at the sorting facility are far more strict than at the diamond mines. That is because diamonds are rare even in the place where they are most abundant in nature.

At the Bultfontein mine in Kimberly, one of three mines still operating in the city where modern diamond mining began in 1866 on a farm owned by the De Beer brothers, it is not uncommon that a miner never sees a diamond during his entire career of breaking rock.

At the Kimberly mines, miners must remove 100 tons of rock to recover 18 carats of diamonds. That means miners dig out an area the size of a six-car garage to extract an eighth of an ounce of stones. Many of the stones are dark, industrial diamonds with limited value. But a single ice-blue whopper can cover the payroll for several months.

Diamonds are found in soft, gray volcanic rock called kimberlite - named after the city. The diamond crystals were formed deep in the earth about three billion years ago under tremendous heat and pressure. They were transported through the earth's mantle about 40 million years ago during a volcanic eruption.

The volcanic kimberlite ``pipes" are shaped like carrots - they become narrower as they get deeper. The diamonds tend to be concentrated closer to the surface. That means a mine becomes less productive as it gets deeper. The three Kimberly mines - all started more than a century ago - are expected to be exhausted in about 10 years.

Most kimberlite pipes are initially exploited as surface mines until the crater becomes too steep to carry the ore out by trucks. The miners then go deeper by building tunnels in the surrounding ``country rock" to attack the soft cylindrical core.

At the Bultfontein mine, the miners work from a maze of tunnels carved out of granite about a half mile underground. The ore body, which had been about 300 yards wide at the surface, is now only about 125 yards in diameter.

At the 845-meter level of the Bultfontein mine, the miners have bored a series of concrete tunnels across the kimberlite pipe. The ore above collapses into the tunnels, where it is mechanically scraped out into a small train that hauls it to a crusher. The crushed rock is hoisted to the surface.

Occasionally, the miners need to dislodge large blocks of kimberlite with dynamite. A siren wails and a thunderous concussion seems to move the air in tunnels. Afterward, a cloud of dust hangs in the cavity where the blast occurred.

A river of dull, gray rubble passes by on a conveyor as the crushed kimberlite makes its way to the surface. It's rather ordinary rock that breaks down once it comes into contact with the air. ``That's the pay dirt," said Bill Fair, a retired miner who now takes visitors on tours. Occasionally a speck of mica glints suggestively, but the miners learn not to pay it any mind.

Fair found a diamond once: It was his third week on the job, and he stumbled across a stone. He worked another 35 years without ever running across another.

De Beers has no idea how many diamonds are pilfered by miners, but productivity went up after the company automated the mining procedure a few decades ago and miners came in less contact with the rock. Where thousands of miners once worked, machines now do most of the heavy lifting.

In the unlikely event that a miner recovers a stone, De Beers pays him a reward equal to 15 percent of the stone's value, an amount calculated to compete with the price the miner could fetch on the black market. Selling the diamond to the boss also carries less risk: It is illegal for anybody other than a licensed dealer to possess a rough diamond in South Africa.

De Beers said it is at more risk of losing diamonds at the treatment facilities on the surface, where the rock is broken down with water and chemicals and the diamonds become concentrated.

Social pressure is also one of the greatest deterrents to theft.

In the close-knit community of Kimberly, rumors quickly spread when a $750-a-month miner begins spending more money than he could possibly earn. Sometimes De Beers fires an employee for suspected pilferage; usually, the unlucky suspect is then ostracized in the community and eventually moves away.