Researchers from Aberdeen University said their find raised the possibility of bigger finds elsewhere.
The discovery is unusual as gold is normally found in "veins" deep in the Earth's crust, which are produced by water flowing at high temperature.
Gold has previously been found in red sandstone in England but not to the consistency discovered in Millport.
The Isle of Great Cumbrae in the Firth of Clyde is off the coast of North Ayrshire.
Professor John Parnell, from Aberdeen University's School of Geosciences, said the find within the rocks from Millport was surprising.
"In undertaking this research I was initially interested in analysing little structures found within common red sandstone rocks which we took from the beach at Millport, which to the naked eye appear as tiny black spots," he said.
"By analysing the structures using a high-powered microscope, we discovered that they contain concentrations of gold, which is unusual because gold normally tends to be concentrated by hot waters rising up through cracks in the Earth.
"In this case the gold has been formed and concentrated in a completely different environment, which raises the possibility of similar structures appearing within red sandstone elsewhere but on a much bigger scale."
Prof Parnell added: "While there is certainly no prospect of a modern-day gold rush in Millport, this analysis is helping us to understand unusual circumstances where gold can be formed, which in the future may even lead to discoveries that are commercially viable."
The Aberdeen University study has been published in the Journal of the Geological Society.

Alien: Covenant, which is out in the UK on Friday, is the sequel to Scott's 2012 Prometheus and stars Michael Fassbender and Katherine Winterston.
The Guardian said the "parasitic space alien has returned for this watchable if unoriginal sci-fi thriller".
But The Telegraph said the film was "grandiose" and "exhilarating".
"To want more Alien after this... would be to want to see something very odd indeed," Robbie Collin continued. "But Covenant leaves the mythos feeling riper and more vitalised than ever."
In The Guardian, Peter Bradshaw went on to say: "The vu has never been so deja: It's a greatest-hits compilation of the other Alien films' freaky moments.
"The paradox is that though you are intended to recognise these touches, you won't really be impressed unless you happen to be seeing them for the first time."
The Independent described it as a "mightily impressive piece of cinematic engineering".
"Every second here is tinged with dark foreboding, with the relentless tension that marked the 1979 original," the paper's Clarisse Loughrey said.
The Mirror was equally effusive, saying: "Scott's final theatrical flourish sends the franchise spinning out in a new direction. This is screamingly great cinema."
In the US, where the movie comes out on 19 May, Variety magazine was firmly on the fence, saying the film was "more of the same, which is both a relief to fans and a letdown to those hoping it might pave new ground".
Its reviewer Peter Debruge concluded: "As acts of creation go, Scott has made an Alien movie for that segment of the audience that has always rooted for the monster."
In the Hollywood Reporter, the verdict was considerably more enthusiastic, saying the film was "gripping through its full two hours and spiked with some real surprises".
"This beautifully made sci-fi thriller will immeasurably boost fan interest in the run of prequels which Scott has recently said will consist of at least two more films until the action catches up to the 1979 original," it said.
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Tiny specks of gold have been found in sandstone rocks from a beach at Millport on the Isle of Great Cumbrae.
[NEXT_CONCEPT]
Ridley Scott's return to directing a film in the Alien franchise has received mixed reviews ahead of its UK and US release.