After panning rivers and lakes on five potential sites, they have subjected the fine dust to microscopic examination and have spotted chromites, often an indicator for the presence of gemstones.
Cambridge Mineral Resources (CMR), a Bristol-based exploration company, has spent the past six months searching sites which include Orkney, Uig on the Isle of Lewis, the area around Ben Hope in the north of Sutherland, the Isle of Colonsay and Lochaber.
Dust samples were sent for examination to a company belonging to Chuck Fipke, the explorer who discovered the multi-billion pound Ekati diamond deposits in Lac de Gras, northern Canada.
The latest hunt is based on the belief that, in Scotland, there are deposits of the type found in the Australian fields. Geophysicists have also become increasingly interested in Scotland because its rock structure is quite similar to that found in Donegal, in the Irish Republic, where Cambridge Mineral Resources is poised to begin drilling for diamonds next year.
"It is early days, but we are enthusiastic about what we have been finding," said David Bramhill, managing director of CMR, who declined to give the precise locations of the potential sites. "In Scotland, unlike in Ireland, there is no proper licensing system for prospecting so we have to be careful not to let others know where we are looking.
"In any case, I would not advise the ordinary public to turn up and start looking. You could trip over a rock containing a sapphire and you would not know what it was."
He said that landowners had so far been very cooperative and he expected that, if gemstones were discovered, royalties might be paid by the mining company at a rate of about 4.5 per cent, turning some Highland crofters into instant millionaires.
The next stage may be an electro-magnetic survey of certain areas.
Mr Bramhill, a veteran of North Sea oil exploration, warned against over- optimism, saying that it was too early to promise success. But he said the company was increasing its exploration budget from pounds 50,000 to pounds 300,000 because of the good results so far.
Dr Graham Smith, principal Scottish minerals geologist at the British Geological Survey, said the work the BGS had carried out for CMR had been encouraging. "I would take what they are doing quite seriously," he said.
Diamonds have never been found in quantity in Britain, although in 1813 what became known as the "Brookeborough Diamond" was discovered in a stream in Co Fermanagh. And in 1995 Britain's largest sapphire, at 9.6 carats worth about pounds 60,000, was discovered at Loch Roag, near Uig on Lewis.