As millions of Irish people at home and abroad celebrate Saint Patrick's Day by pinning shamrocks to their clothes and downing the odd pint or two, their Celtic cousins across the sea in Scotland have shattered a couple of myths about one of Ireland's most iconic tourist attractions.

The Blarney Stone – famous for giving you the "gift of the gab" if you kiss it – is 100% Irish, according to researchers at Glasgow University.

For centuries, legends have abounded about the origins of the stone, which some have claimed was hewn from Stonehenge or sent over as a gift from the Scots by Robert the Bruce after victory at the battle of Bannockburn in 1314. But the secret of the stone has been unravelled after the discovery of a unique 19th-century microscopic slide taken from the rock at Blarney Castle, near Cork.

Geologists at the University of Glasgow's Hunterian Museum can reveal the true nature of the Stone after studying the historic microscope slide, containing a slice of the stone ground so thin that it is transparent to light. Their analysis indicates the Blarney is a limestone, made of the mineral calcite, and containing recrystallised and slightly deformed fragments of fossil brachiopod shells and bryozoans – all of which are unique to the region where it is based.

Dr John Faithfull, curator at the Hunterian museum, said: "This strongly supports views that the stone is made of local carboniferous limestone, about 330m years old, and indicates that it has nothing to do with the Stonehenge bluestones, or the sandstone of the current 'Stone of Destiny', now in Edinburgh Castle."

The Hunterian has about 40,000 geological microscope slides, and the older slides are mostly catalogued in handwritten ledgers. The Blarney Stone slide had escaped notice until now, but it was spotted in the catalogues during a digitisation programme being carried out by Becky Smith, a Museums Galleries Scotland intern.

The slide is part of the rock and mineral thin-section collection put together by Professor Matthew Forster Heddle, of St Andrews, one of the giants of 19th-century geology and chemistry in the UK.

Faithfull said: "It was probably made between 1850 and 1880, during the period when new microscope techniques began to revolutionise our understanding of rocks, and how they form.

"He was a pioneer in the use of these techniques to investigate the rocks and minerals of Scotland, and elsewhere. He also seems to have managed to obtain fragments of a number of important historical stones: the collection also includes slides cut from several of the stones at Stonehenge. This was not vandalism – it was bringing the latest scientific tools to bear on the origins of these monuments.

"We don't know how Prof Heddle got the Blarney microscope slide, or whether he had it made himself, but he was a major scientific figure, with excellent contacts, and was always keen to acquire interesting samples for scientific investigation. However, in this case, he doesn't seem to have published anything about the stone."

During the mid-19th century, Blarney Castle was an uninhabited semi-ruin, but visiting the stone was popular, and Heddle or another geologist or antiquarian must have collected a sample. At that time, the stone could only be accessed by hanging upside down from the battlements, so using a hammer to take a sample would have been a difficult acrobatic feat.

Faithfull added: "Very few pieces of the Blarney Stone seem to exist outside Blarney Castle. Apart from our microscope slide, the only other one I'm aware of is in a monument at the University of Texas. However, this object seems to have its origins in a beer-fuelled party, and the genuineness of the fragment must be in doubt.

"The Blarney Stone is famous for bestowing the gift of eloquence on those who kiss it. We don't know if kissing the microscope slide would have the same effect, although I have tried it."