Diamonds and sapphires kept at the Czech National Museum in Prague and thought to be worth millions have been found to be fake.
A 5-carat diamond acquired by the museum in 1968 has been revealed to be a worthless piece of cut glass during an audit by concerned curators.
Meanwhile a 19-carat sapphire bought by the museum for 200,000 crowns (£7,000) in 1978 has been found to be a cheap imitation worth only a fraction of that sum.
Half of the museum's collection of rubies are also thought to be fakes.
Liars: A diamond in the Prague National Museum's collection was found to be a worthless piece of cut glass during an inspection by curators
A sapphire bought by the museum for 200,000 crowns (£7,000) back in 1978 has been found to be synthetic, and worth only a fraction of that sum
It is not known how the fakes have come into the museum's possession, and while it is possible that they were not properly checked at the time of acquisition, they may also have been swapped by criminals and the real gems stolen.
The discovery, first reported by HlidaciPes.org, was made during an inspection of some of the museum's 5,000 precious stones and minerals, kept under lock and key in the Czech Republic's capital.
Christopher A Marinello, a lawyer and director of Art Recovery International, which specialises in the recovery of stolen, missing and disputed cultural property, described the case as 'pretty fantastic'.
He said: 'It's fifty-something years of potential suspects. I don't know how they're going to pinpoint who's responsible.
'Even a honeymoon couple who buy an engagement ring have to be careful that what they saw and what actually gets puts on the finger is the same ring.
'Every time a stone gets cleaned, you have to have it done by somebody who's not going to swap out the gems.'
The embarrassing discoveries were made during an inspection of some of the museum's 5,000 precious stones and minerals
He added: 'These things are often done by insiders who have significant access to the objects, plenty of time,… and they can hide their crimes without anyone seeing them.'
Museums worldwide will be unnerved by the Prague case, he said, warning that many more such cases are likely to be discovered as technology improves and more institutions start having their collections reviewed forensically.
'What we have here is still a sapphire, but it is not a natural stone as was documented when the museum gained it for its collection in the 1970s,' Ivo Macek, head of the museum's precious stones department, told Radio Praha
'It was artificially created so it does not have the value we thought it did. It was acquired for 200 thousand crowns and today it would have been worth tens of millions.
'And what we thought to be a 5-carat diamond was in fact plain glass given a diamond cutting.'
Scratching the surface: The discovery was made during an inspection of the Prague National Museum's 5,000 precious stones and minerals
The museum says they do not know how the fakes ended up in their collection, and is carrying out an investigation.
It is not known if the stones were already fake when they were acquired by the Museum in the 60s and 70s.
It is also not known if staff at the time did not have the expertise to catch the fakery, or if real gems were in fact acquired, and these have since been replaced by thieves.
The person in charge at the time has since passed away, and Radio Praha reports that the collection have been kept 'under lock and key' from the start.
So far, the museum has been able to check 400 of their 5,000 stones, with the full review of the collection expected to be completed by 2020.
The musem's deputy director Michal Stehlík, attempted to play down the issue when speaking to the radio station.
He said: 'When you have a collection of 20 million artifacts then a certain fraction of that may prove to be problematic. These things happen.
'So we will push ahead with the audit and I think we may even organize an exhibition of fakes in this and other world museums when it is concluded in 2020.'
BIRTH OF THE REPUBLIC: CZECH HISTORY
The Czech Republic can trace its roots back to the 9th century when it was founded as the Duchy of Bohemia under the Great Moravian Empire.
Following the collapse of that empire it became part of the Holy Roman Empire, and was then absorbed by the Austro-Hungarian Empire.
Czechoslovakia came into existence as an independent country in 1918 following the end of the First World War and the collapse of world empires.
During the Second World War the Czech part of the country was occupied by the Nazis, with the Slovak part becoming the Slovak Republic.
In 1945 the entire country was liberated by the US and Russia before electing a communist government the following year.
A 1948 coup d'état established the country as a one-party state under Soviet influence, and was occupied by the Soviets from 1968 after a failed uprising.
The country would remain under occupation until 1989 when the Velvet Revolution ended the one-party system and reinstated the market economy.
On 1 January 1993 Czechoslovakia peacefully dissolved, becoming the independent states of the Czech Republic and Slovakia.