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.
Tiny specks of gold have been found in sandstone rocks from a beach at Millport on the Isle of Great Cumbrae.