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A rare silver coin minted during the reign of a famous Viking king has been discovered in a massive hoard in Norway that now comprises 5,500 coins and counting.The coin is inscribed with the name of the Norwegian king Olaf Haraldsson, who ruled from 1015 to 1028 and, as St. Olaf or "Olaf the Holy," is considered the national saint of Norway.Fewer than 20 coins minted during Olaf's reign have ever been discovered, and the newfound coin is a "national treasure," according to a statement from the University of Oslo's Museum of Cultural History."Here we have a coin bearing the king's portrait and a cross, with another cross on the reverse," University of Oslo coin expert Svein Gullbekk said in the statement. "It is a Christianisation coin on a grand scale."Olaf is credited with establishing Christianity in Norway — an early step toward its full conversion about a generation later."We can see how King Olaf used coinage to legitimise the new faith and to establish himself as a Christian king of European standing," Gullbekk said. "This is branding."Viking kingHaraldsson learned about Christianity when he joined Viking raids against England and France as a young man, and he was baptized in the French city of Rouen in about 1013, according to the statement.He became Norway's king in 1015, and in about 1024 proclaimed that only Christianity would be legal, according to the University of Bergen. Much of Norway, however, remained faithful to the Norse gods during his rule, especially around the central Trøndelag region and in the eastern parts of the country.The front of the silver coin portrays "Olaf the Holy," who is now Norway's national saint. (Image credit: Lill-Ann Chepstow-Lusty / Museum of Cultural History)But Olaf was made a Christian saint shortly after his death in 1030. He is now recognized as one of the first Christian kings of Norway and the first Norwegian king to issue coins in his own name.The newfound coin is part of the Mørstad Hoard, which was discovered near the village of Rena in April. At that time, the buried hoard was described as the largest ever found in Norway and was estimated to contain almost 3,000 silver coins. However, subsequent research has determined that it includes anywhere from 5,500 to more than 6,000 coins, the statement said. Mistaken identityAnne Skogsfjord, a museum researcher charged with cataloging the coins, thought the Olaf coin was an Anglo-Saxon penny at first. But then she read the inscription and realized it was minted in Norway under Olaf."I could see that it read ONLAF R+ NORMANORV. … Could this really be a coin of Olaf Haraldsson that I was holding?" Skogsfjord said in the statement. "And it was! It was a very special moment."Olaf Haraldsson was one of Norway's first Christian kings; the other side of the coin features a cross. (Image credit: Lill-Ann Chepstow-Lusty / Museum of Cultural History)The coin was struck between about 1024 and 1028, near the end of the Viking Age, and circulated for 20 to 25 years before someone deposited it in the hoard, the statement said. Related storiesDirty 'button' unearthed by metal detectorist turns out to be a rare 900-year-old coin from Norway's last Viking king, Magnus Barefoot'Biggest surprise of my life': Homeowner discovers Denmark's largest Viking Age silver hoard in their backyard'There was something bigger in the ground here': Metal detectorist in Germany discovers giant Roman-era hoard from time of emperor HadrianAlthough the minting of the coin was a political and religious act, the coin also seems to have been used as currency; both sides bear numerous small cut marks, which was a common way of determining whether a coin was made completely of silver.Gullbekk said Haraldsson himself had decided what the coins would look like and may have been present when they were minted."It is entirely possible that this very coin from Mørstad was once held in the king's own hand," he said in the statement.See how much you know about ancient norsemen with our Viking quiz!