In Australia: 'diamonds are a businessman's best friend'

''Up here, diamonds are a businessman's best friend,'' says an innkeeper in the remote Australia outback.

Not far from the inn, Australia's first diamond mine is slowly taking shape. The glittering wealth already unearthed means the mine - located in the rugged north of mineral-rich Western Australia - probably will open late next year.

The impact of the mine on the world diamond market is expected to be enormous. Test yields are said to show more promise than Africa's two top mines.

The deposit is expected to add 20 million carats a year to the world's present 50-million-carat annual production. (Its actual value will amount to just 8 percent of world production - but that is still a large chunk.)

Equally significant is the prospect that Australia might market its diamonds independently instead of through De Beers, the South African mining cartel that controls the diamond trade. Independent action could disrupt De Beers' careful manipulation of supply and demand, flood the market with diamonds, and cause diamond prices to plummet, Atlantic Monthly magazine reports.

Locally the mine - reachable only by helicopter until a road is completed - is producing a big impact, too.

Little Derby, Australia, for example, is growing by leaps and bounds.

''We've become a boom town all right,'' says John Thompson-Dray, manager of the Derby Boab Inn.

''Derby was a sleepy little place a couple of years ago,'' he says. ''Now diamond men from all over the world are visiting. And Esso and Mobil are looking for oil close by - so oilmen are often in town, with plenty to spend.''

Rooms in the two local hotels and one motel are usually taken. Restaurants and bars are crammed with men who talk diamonds - and gold and oil.

Announcement in 1979 of the diamond discovery sparked a wave of Australian diamond fever - a mineral rush the like of which hasn't been seen since the country's nickel boom in the early 1970s in which giant fortunes were won and lost.

Armchair prospectors are rushing diamond shares, and adventurous individuals are following companies into the region, seeking their fortunes.

About 500 diamond prospecting licenses have been issued.

Courts have had to settle some disputed claims. So far, however, individual prospectors have found only limited amounts of diamonds.

''Diamond hunting is an expensive game these days for which only the big companies are properly equipped,'' says Dr. P. E. Playford, acting geology chief with the Western Australian mines department.

''The big bonanza, so far, has been for the helicopter companies,'' he reveals. ''Helicopters have been in incredible demand for claim-pegging - and also for carrying out prospecting equipment.''

The biggest investor in the diamond project is CRA, Ltd., which is linked to Britain's Conzinc Riotinto group. It has almost 57 percent of the shares.